fix: QA remediation P1-A/P1-B/P1-C/P2 for v0.22.1 sync-mode

P1-A: wrap wp_schedule_single_event(GENERATE_ACTION) + fire_cron_now() in
on_orchestrate_tick() with 'if ( ! self::$sync_mode )' guard.
Mirrors the existing guard in on_generate_tick(). Prevents a background
wp-cron process spawning in parallel with the VPS --sync loop and
prematurely finalizing the run lock before Article_Worker completes.

P1-B: add two unit tests to GenerationCheckpointRunnerTest.php:
- test_orchestrate_tick_in_sync_mode_does_not_schedule_background_event:
  asserts sync_mode=true suppresses GENERATE_ACTION schedule + spawn_cron.
- test_generate_tick_async_mode_schedules_generate_action:
  asserts sync_mode=false (async path) still reschedules GENERATE_ACTION.

P1-C: create root CONTEXT.md with domain glossary (run, run_stages,
checkpoint, idea queue, Generation_Lock) and execution mode reference
(async wp-cron vs --sync VPS-driven).

P2: update ARCHITECTURE.md §25 Entry Points table with the
'wp prautoblogger generate --sync' VPS path row.

Agent-Session: prab-reliability-remediation-20260622

CONTEXT.md

@@ -0,0 +1,77 @@
+# PRAutoBlogger — Domain Glossary
+
+Quick reference for the generation pipeline domain model. For full design rationale
+see `ARCHITECTURE.md` (§21–§25) and `CONVENTIONS.md`.
+
+---
+
+## Core Domain Terms
+
+**run**
+A single end-to-end article generation cycle: orchestrate → generate idea(s) →
+finalize. Represented as a row in `wp_prautoblogger_runs` (status, cost ceiling,
+timestamps). One run may produce multiple articles if the daily idea-count > 1.
+
+**run\_stages** (`wp_prautoblogger_run_stages`)
+Sub-steps within a run (research, analysis, generation, publish). Each stage has
+its own status and can be replayed independently via `Rerun_Executor`. Generation
+checkpoint runs populate stages as Article_Worker progresses.
+
+**checkpoint**
+The persistence boundary between pipeline ticks. After each tick the idea queue
+and run state are written to `wp_options` so a process kill cannot lose work. The
+next tick reads the queue option and continues. See §25 of ARCHITECTURE.md.
+
+**idea queue** (`prautoblogger_article_queue` option)
+Serialized array of `Article_Idea` objects written by `on_orchestrate_tick()` and
+consumed one-at-a-time by successive `on_generate_tick()` calls. Deleted on
+finalize or cleanup.
+
+**Generation\_Lock** (`wp_prautoblogger_generation_lock` or `wp_options`)
+DB mutex that prevents two concurrent runs. Acquired at the start of
+`on_orchestrate_tick()`, released by `Helpers::finalize()` or on error/halt.
+
+---
+
+## Execution Modes
+
+### Async (wp-cron driven)
+
+Default path for the Board "New Article" button and the plain
+`wp prautoblogger generate` WP-CLI command.
+
+1. `kick_off()` schedules `prautoblogger_gen_orchestrate` and calls `fire_cron_now()`.
+2. `on_orchestrate_tick()` runs in a background PHP process, schedules
+   `prautoblogger_gen_tick`, and calls `fire_cron_now()` to spawn the next process.
+3. Each `on_generate_tick()` spawns the next until the queue drains.
+
+Limitation: Hostinger kills background PHP processes at ~10 min. Orchestrate tick
+(LLM-free) and individual generate ticks (one LLM call each) are both well within
+this limit; the checkpoint design is the mitigation.
+
+### Sync (VPS-driven / `--sync`)
+
+Used by the Coolify KVM8 VPS systemd timer via `infra/vps/orchestrate-generation.sh`
+in `peptide-e2e`. Introduced v0.22.1.
+
+1. `run_sync()` sets `Generation_Checkpoint_Runner::$sync_mode = true`.
+2. Calls `on_orchestrate_tick()` **in-process** (no background spawn).
+3. Loops calling `on_generate_tick()` directly until queue empty or terminal state.
+4. `$sync_mode = true` suppresses all `wp_schedule_single_event` +
+   `fire_cron_now` calls inside both tick methods, preventing a competing
+   background cron chain from forming in parallel.
+
+The VPS SSH session is held open; the process is not subject to Hostinger's
+background-kill constraint.
+
+---
+
+## Table Shorthand
+
+Throughout the codebase `wp_prautoblogger_*` tables are referenced by their
+`$wpdb->` property names (e.g. `$wpdb->prab_runs`, `$wpdb->prab_run_stages`).
+Full table names are defined in `class-activator.php`.
+
+---
+
+*See also: `ARCHITECTURE.md` §25 (checkpoint design), `CONVENTIONS.md` (coding rules).*