docs: add Git Workflow section to CONVENTIONS.md

Documents the PR-gated review process enforced at the agent layer.
Free-plan workaround since branch protection requires GitHub Pro.

CONVENTIONS.md

@@ -141,3 +141,49 @@ The plugin currently talks to OpenRouter, which proxies to multiple LLM provider
 ## Type safety
 
 All PHP files use `declare(strict_types=1)`. All public and private methods have typed parameters and return types. This catches type errors at call time rather than at runtime and gives AI agents structural information for reasoning about code.
+
+## Git Workflow — PR-Gated, Soft-Enforced
+
+This repo is private on GitHub's free plan, which does not support branch protection or rulesets. The review gate is enforced at the agent layer, not server-side. Every agent and human contributor follows these rules; the `.github/workflows/main-push-audit.yml` tripwire opens an audit issue on any direct push to `main` that did not come from a merged PR.
+
+### Rules
+
+1. **Never push to `main` directly.** Every change lands via a pull request that the maintainer merges. No self-merging. No force-pushing to `main`.
+
+2. **Branch naming**: `claude/<scope>-<YYYYMMDD>` for agent-authored work, `fix/<scope>` or `feat/<scope>` for human-authored. The scope is a 1–3 word kebab-case description.
+
+3. **Commit trailer**: every commit authored by an agent must end with an `Agent-Session:` trailer so commits can be correlated back to the conversation that produced them, even though all commits share the `peptiderepo` bot identity.
+
+   ```
+   feat: add per-request cost tracking
+
+   Agent-Session: cowork-2026-04-14-cost-audit
+   ```
+
+4. **PR description template**: every PR description covers
+   - **What changed** (one paragraph)
+   - **Why** (motivation or incident link)
+   - **Risk flags** (schema changes, API contract changes, cost impact, compatibility)
+   - **Test plan** (what was run locally, what to smoke-test after merge)
+
+5. **Emergency push exception**: if a situation genuinely requires pushing to `main` without a PR (site down, CI broken, one-line hotfix), surface it in the chat before doing it. Every emergency push gets a follow-up PR that commits the same changes through the normal flow so git stays the source of truth. The tripwire will open an audit issue — close it with a comment explaining why.
+
+### Opening a PR from an agent session
+
+The `gh` CLI is not installed in the Cowork sandbox. Use `curl` with the PAT from the workspace `.env.credentials`:
+
+```bash
+GH_TOKEN=$(grep "^GITHUB_PAT=" "$WORKSPACE/.env.credentials" | cut -d= -f2)
+
+git push -u origin HEAD
+
+curl -s -X POST -H "Authorization: token $GH_TOKEN" \
+  -H "Content-Type: application/json" \
+  "https://api.github.com/repos/peptiderepo/<repo>/pulls" \
+  -d "$(jq -cn --arg title "<title>" --arg head "<branch>" --arg body "<body>" \
+      '{title:$title, head:$head, base:"main", body:$body}')"
+```
+
+### When this changes
+
+If the peptiderepo GitHub account is ever upgraded to Pro (or the repo goes public), replace this soft-enforcement section with a note pointing to the real branch protection rules in repo settings.