docs(l0): parameterize hard gates on the project's integration trunk

Per Jason's Option-A ruling on #1216: the constitution supplies the general
framework; the project declares its own flow. Defines the integration trunk
once in CONSTITUTION.md Hard Gates — declared by exactly one
'Integration trunk: <branch>' line in the project's root AGENTS.md, default
'main', value bound by git check-ref-format --branch semantics, malformed or
multiple declarations are a hard stop, one trunk only, no gate relaxation —
and binds gates 5/15, E2E-DELIVERY, CODE-REVIEW, ORCHESTRATOR(-PROTOCOL),
BOOTSTRAP provisioning, and the defaults AGENTS session closure to that term.
CI-CD-PIPELINES YAML examples stay on the default trunk with a substitution
note.

Known mechanized consumer left as tracked follow-up on #1216: pr-merge.sh:133
hardcodes a {main,next} base allowlist — fail-closed (refuses exotic trunks,
cannot misroute), to be parameterized with red-first tests separately.

Refs #1216

Co-Authored-By: Claude Fable 5 <[email protected]>
Claude-Session: https://claude.ai/code/session_01WYgWocp36goy8hj2ui6ps1

packages/mosaic/framework/guides/CODE-REVIEW.md

@@ -10,9 +10,10 @@ If implementation diverges from `docs/PRD.md` or `docs/PRD.json` without PRD upd
 
 Merge strategy enforcement (HARD RULE):
 
-- PR target for delivery is `main`.
-- Direct pushes to `main` are prohibited.
-- Merge to `main` MUST be squash-only.
+- The integration trunk is the branch the project's root `AGENTS.md` declares (default: `main`) — see `CONSTITUTION.md` Hard Gates.
+- PR target for delivery is the integration trunk.
+- Direct pushes to the integration trunk are prohibited.
+- Merge to the integration trunk MUST be squash-only.
 - Use `~/.config/mosaic/tools/git/pr-merge.sh -n {PR_NUMBER} -m squash --expect-head {approved_full_sha}` (or PowerShell equivalent).
 
 ## Review Checklist
@@ -114,8 +115,8 @@ Use `~/.config/mosaic/templates/docs/DOCUMENTATION-CHECKLIST.md` whenever code/A
 # List the issue being addressed
 ~/.config/mosaic/tools/git/issue-list.sh -i {issue-number}
 
-# View the changes
-git diff main...HEAD
+# View the changes (diff against the integration trunk; default: main)
+git diff {integration_trunk}...HEAD
 ```
 
 ### Providing Feedback
@@ -151,4 +152,4 @@ This pattern appears in 3 places. A shared helper would reduce duplication.
 2. If changes requested, assign back to author
 3. If approved, note approval in issue comments
 4. For merges, ensure CI passes first
-5. Merge PR to `main` with squash strategy only
+5. Merge PR to the integration trunk with squash strategy only