feat: M4 chained-cron checkpoint generation + board button (v0.21.0)

## Board 'New Article' button
- templates/admin/board-page.php: add #prab-board-generate-now button + status div
- includes/admin/class-board-page.php: inject generateNonce + i18n strings into prabBoard;
  enqueue companion board-generate.js
- assets/js/board-generate.js: NEW – board button AJAX handler (fires
  prautoblogger_generate_now, resets board backoff on success)
- assets/js/board.js: applySnapshot() re-enables button when no active runs

## Chained-cron checkpoint machine (retires SSH workaround)
- includes/core/class-generation-checkpoint-runner.php: NEW (295 lines)
  Tick 1 (prautoblogger_gen_orchestrate): orchestrate_only() → persist queue → schedule Tick 2
  Tick 2..N (prautoblogger_gen_tick): pop one idea → Article_Worker → reschedule or finalize
  Budget/halt contract + lock release on all exit paths
- includes/class-executor.php: on_manual_generation() replaced with 1-line delegate to
  Generation_Checkpoint_Runner::on_orchestrate_tick() (SSH workaround retired)
- includes/class-prautoblogger.php: register ORCHESTRATE_ACTION + GENERATE_ACTION hooks
- includes/core/class-pipeline-runner.php: expose orchestrate_only() public wrapper
- includes/core/class-pipeline-status.php: add write_initial() + write_error() helpers

## WP-CLI command
- includes/admin/class-wp-cli-commands.php: wp prautoblogger generate → kick_off()

## IA / perf pass (settings-backed pagination + DB indexes)
- includes/admin/class-board-data-provider.php: replace hardcoded posts_per_page=20 with
  get_option('prautoblogger_board_column_limit', DEFAULT)
- includes/admin/class-board-gen-log-query.php: replace LIMIT 10/20 literals with settings-backed values
- includes/admin/class-ideas-browser-query.php: NEW – query helpers extracted from Ideas_Browser
  (300-line rule split): query_ideas(), get_type_counts(), load_idea_data(),
  find_post_by_run_id(), update_idea_stage()
- includes/admin/class-ideas-browser.php: delegates all query work to Ideas_Browser_Query
- includes/class-activator.php: set_default_options() adds board_column_limit + ideas_per_page
- includes/admin/class-admin-page.php: add new settings to numeric sanitization list
- prautoblogger.php: PRAUTOBLOGGER_DEFAULT_BOARD_COLUMN_LIMIT=20, PRAUTOBLOGGER_DEFAULT_IDEAS_PER_PAGE=30
- includes/class-schema-installer.php: indexes on generation_log + analysis_results (db 1.4.0)
- includes/class-pipeline-schema-installer.php: KEY started_at on runs table

## Guard P2s
- scripts/check-constants.php: --self-test mode + inline CONSTANTS_GUARD_WINDOW comment
- .github/workflows/ci.yml: check-constants-self-test step after main constants check

## PHPUnit test
- tests/unit/Core/GenerationCheckpointRunnerTest.php: NEW (227 lines)
  4 tests: kick_off schedules+transient, empty-queue finalizes, halted-run aborts+cleanup,
  multi-idea pops-one-and-reschedules

## Docs + version
- prautoblogger.php: Version 0.21.0, PRAUTOBLOGGER_DB_VERSION 1.4.0
- ARCHITECTURE.md: §25 Chained-Cron Checkpoint Generation Path
- CHANGELOG.md: v0.21.0 entry
- INFORMATION-ARCHITECTURE.md: NEW – admin pages, entry points, DB tables, options, CLI
- uninstall.php: purge prautoblogger_gen_orchestrate + prautoblogger_gen_tick cron hooks

Agent-Session: phase2-admin-m4-20260614

CHANGELOG.md

@@ -10,6 +10,61 @@ and this project uses [Semantic Versioning](https://semver.org/).
 ### Internal
 - CI constants guard (`scripts/check-constants.php`): new required CI step (`Check PRAUTOBLOGGER_* constants`) fails the build when any `PRAUTOBLOGGER_*` constant referenced in shipped plugin PHP is neither defined in `prautoblogger.php` nor `defined()`-guarded at the reference site. Prevents a recurrence of the v0.19.0 admin-500 regression class.
 
+## [0.21.0] - 2026-06-14
+
+### Added
+- **Board “New Article” button (Phase 2 admin M4)**. Manual generation is now
+  accessible from the kanban board (not just the Settings page). The button fires
+  the existing `prautoblogger_generate_now` AJAX action; the board status area
+  shows generation progress via the existing transient poller. Requires
+  `manage_options` capability.
+- **Chained-cron checkpoint machine** (`PRAutoBlogger_Generation_Checkpoint_Runner`).
+  Replaces the monolithic `on_manual_generation()` synchronous path with a
+  two-tick checkpoint machine so Hostinger’s PHP-process kill cannot abort a
+  mid-pipeline run:
+  - Tick 1 (`prautoblogger_gen_orchestrate`): collect → analyze → score ideas,
+    persist to `prautoblogger_article_queue` option, schedule Tick 2.
+  - Tick 2…N (`prautoblogger_gen_tick`): pop ONE idea, generate one article,
+    reschedule or finalize. Persists consumed queue BEFORE generating to prevent
+    orphan-recovery duplication on restart.
+  - Budget / halt: `Cost_Tracker::is_budget_exceeded()` checked per tick;
+    a halted run aborts all remaining ticks without calling `Article_Worker`.
+- **WP-CLI `wp prautoblogger generate` command**. Routes through the same
+  chained-cron checkpoints as the board button. Replaces the retired SSH-only
+  `wp eval 'do_action("prautoblogger_manual_generation")'` workaround.
+- **Settings-backed board pagination** (perf pass). All hardcoded LIMIT / `posts_per_page`
+  values in board queries replaced with two new settings:
+  - `prautoblogger_board_column_limit` (default 20, constant `PRAUTOBLOGGER_DEFAULT_BOARD_COLUMN_LIMIT`)
+  - `prautoblogger_ideas_per_page` (default 30, constant `PRAUTOBLOGGER_DEFAULT_IDEAS_PER_PAGE`)
+- **DB indexes** (via `PRAUTOBLOGGER_DB_VERSION` → `1.4.0`):
+  - `generation_log`: `KEY run_id_post_id`, `KEY created_status` (board query + failed-cards).
+  - `analysis_results`: `KEY analyzed_type` (ideas-browser pagination + type filter).
+  - `runs`: `KEY started_at` (articles-list pagination `ORDER BY started_at DESC LIMIT n OFFSET m`).
+- **Query helper split**: `PRAutoBlogger_Ideas_Browser_Query` (new `class-ideas-browser-query.php`)
+  carries static query helpers extracted from `class-ideas-browser.php` to enforce the
+  300-line rule (CONVENTIONS §1).
+- **Guard P2s** (`scripts/check-constants.php`):
+  - Added `--self-test` mode: synthesizes a missing constant and asserts the scanner catches it.
+  - Added inline comment documenting `CONSTANTS_GUARD_WINDOW = 4`.
+  - Wired `--self-test` as a CI step after the main constants check.
+- **PHPUnit test** `tests/unit/Core/GenerationCheckpointRunnerTest.php`: tick-by-tick
+  state-machine proof (kick_off schedules only, empty queue finalizes, halted run aborts,
+  multi-idea queue pops one and reschedules).
+- **Pipeline_Status** now exports `write_initial()` and `write_error()` for the
+  checkpoint runner to set the status transient before the first cron tick fires.
+
+### Changed
+- `Executor::on_manual_generation()` now delegates to
+  `Generation_Checkpoint_Runner::on_orchestrate_tick()` instead of running the full
+  pipeline synchronously. The method body is 1 line; the synchronous path is retired.
+- `Pipeline_Runner::orchestrate_only()` added as a public wrapper exposing the private
+  `orchestrate()` method for use by `Generation_Checkpoint_Runner`.
+
+### Removed
+- **SSH-only workaround retired**: `wp eval 'do_action("prautoblogger_manual_generation")'` no
+  longer fires the pipeline synchronously in one PHP process. Use `wp prautoblogger generate`
+  instead (routes through the chained-cron checkpoint machine). See ARCHITECTURE.md §25.
+
 ## [0.20.1] - 2026-06-12
 
 ### Fixed