docs: apply PR #82 review feedback (gemini)

- CONTRIBUTING.md: update verify-plugin.sh check count from 34 to 57
  (actual current count; was already 45 in v1.3.0 per CHANGELOG line 253)
- CONTRIBUTING.md: align agent count in PR checklist with verify-plugin.sh
  (143 -> 144, matches scripts/verify-plugin.sh line 49)
- CONTRIBUTING.md: rewrite "Release process" section to describe the
  release-please automated flow (replaces the contradictory manual
  tag/bump steps that conflicted with the new commit-message convention
  introduced earlier in this same file)
- CONTRIBUTING.md: PR checklist now points contributors at the
  hand-maintained plugins/preview-forge/CHANGELOG.md and warns against
  hand-editing the release-please-managed root CHANGELOG.md / manifests

Refs PR #82 review comments

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: sgwannabe

CONTRIBUTING.md

@@ -49,7 +49,7 @@ for the major bump.
 ```bash
 git clone https://github.com/Two-Weeks-Team/PreviewForgeForClaudeCode
 cd PreviewForgeForClaudeCode
-bash scripts/verify-plugin.sh   # 34 checks
+bash scripts/verify-plugin.sh   # 57 checks (Pass: 57 / Fail: 0)
 ```
 
 Install the plugin under development locally:
@@ -118,13 +118,16 @@ fix the code, not the proposal (or justify the divergence in the PR).
 
 ## PR checklist (auto-enforced by CI)
 
-- [ ] `bash scripts/verify-plugin.sh` passes (all 34 checks)
-- [ ] Agent count still 143, or CHANGELOG + proposal updated explicitly
+- [ ] `bash scripts/verify-plugin.sh` passes (all 57 checks)
+- [ ] Agent count still 144, or CHANGELOG + proposal updated explicitly
 - [ ] All agents use `model: opus` (no Sonnet/Haiku mixing)
 - [ ] No 3rd-party service dependencies introduced
 - [ ] Layer-0 rules respected (`plugins/preview-forge/methodology/global.md`)
-- [ ] Version bumped if release-worthy (both manifests)
-- [ ] `CHANGELOG.md` entry
+- [ ] Conventional Commit message with **semantic scope** (release-please
+      autobumps semver — do **not** hand-edit version in manifests or root
+      `CHANGELOG.md`)
+- [ ] `plugins/preview-forge/CHANGELOG.md` entry (the hand-maintained
+      plugin changelog; root `CHANGELOG.md` is release-please generated)
 
 ## Code of conduct
 
@@ -134,13 +137,25 @@ should treat each other as collaborators, not as Devil's Advocates.
 
 ## Release process
 
-1. PR merged to `main`
-2. `version` bumped in both `.claude-plugin/marketplace.json` and
-   `plugins/preview-forge/.claude-plugin/plugin.json`
-3. `CHANGELOG.md` entry added under `## [X.Y.Z]`
-4. `git tag vX.Y.Z && git push origin vX.Y.Z`
-5. GitHub Actions `release.yml` automatically creates the GitHub Release
-   with notes extracted from CHANGELOG
+Releases are automated via **release-please**. Contributors do **not**
+hand-bump versions or hand-tag releases.
+
+1. PR with Conventional Commit messages (semantic scope) merges to `main`.
+2. release-please opens / updates a `chore(main): release X.Y.Z` PR that
+   accumulates pending bumps from the commit history.
+3. When that release PR is merged, release-please:
+   - bumps `version` in `.claude-plugin/marketplace.json` and
+     `plugins/preview-forge/.claude-plugin/plugin.json`,
+   - regenerates the **root** `CHANGELOG.md`,
+   - creates the `vX.Y.Z` git tag, and
+   - publishes the GitHub Release with notes extracted from the changelog.
+4. `plugins/preview-forge/CHANGELOG.md` is **hand-maintained** in feature
+   PRs — add an entry under `## [X.Y.Z]` describing the user-visible
+   change. release-please does not touch this file.
+
+If your change is release-worthy, the only contributor action is writing
+a clean Conventional Commit and updating
+`plugins/preview-forge/CHANGELOG.md`.
 
 ## Questions