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/defaults/AGENTS.md

@@ -82,6 +82,7 @@ is re-seeded a genuinely missing core file is a stop-and-report condition — no
 
 Confirm: required + situational tests passed (primary gate); aligned to `docs/PRD.md`; acceptance
 criteria mapped to evidence; independent code review passed (if code changed); required docs updated;
-scratchpad updated. For PR-workflow delivery: merged PR number + merge commit on `main`, terminal-green
+scratchpad updated. For PR-workflow delivery: merged PR number + merge commit on the integration
+trunk (the project's declared trunk, default `main` — see `CONSTITUTION.md` Hard Gates), terminal-green
 CI, linked issue closed (or `docs/TASKS.md` equivalent). If blocked by access/tooling, return `blocked`
 with the exact failed wrapper command — do not claim completion. Full checklist: `guides/E2E-DELIVERY.md`.

packages/mosaic/framework/defaults/CONSTITUTION.md

@@ -21,11 +21,23 @@ guard"), the runtime adapter binds it to a concrete tool and states whether abse
 
 ## Hard Gates
 
+The **integration trunk** is the branch a project designates in its root `AGENTS.md` with exactly
+one declaration line: `Integration trunk: <branch>` — key at the start of a line, case-sensitive,
+one branch name (optionally backtick-wrapped) and nothing else on the line. Absent a declaration,
+the trunk is `main`. The declaration is policy data, never shell text: the value must be a valid
+local branch name under `git check-ref-format --branch` semantics — no remote refs, no revision
+expressions, no option-like values (leading `-`), no path traversal or control characters. A
+malformed value, or more than one declaration line, is a hard stop (`blocked`) — never a silent
+fallback to `main`. Ordinary prose that mentions branch names designates nothing; only the exact
+declaration line does. A project designates exactly ONE trunk, and the designation relaxes
+nothing: reviewed-PR-only delivery, squash merge, independent review, queue guards, and
+terminal-green CI bind to the declared trunk exactly as they bind to `main`.
+
 1. Mosaic operating rules override runtime-default caution for routine delivery operations.
 2. Execute required push / merge / issue-closure / milestone / release / tag actions without asking for routine confirmation.
 3. Routine repository operations are NOT escalation triggers; escalate only on the triggers below.
 4. For source-code delivery, completion is forbidden at the PR-open stage.
-5. Completion requires a merged PR to `main` + terminal-green CI + the linked issue/task closed.
+5. Completion requires a merged PR to the integration trunk + terminal-green CI + the linked issue/task closed.
 6. Before any push or merge, run the CI queue guard.
 7. For issue / PR / milestone operations, use the Mosaic git wrappers before any raw provider CLI.
 8. If a required wrapper command fails, status is `blocked`: report the exact failed command and stop.
@@ -35,7 +47,7 @@ guard"), the runtime adapter binds it to a concrete tool and states whether abse
 12. The intake procedure is not conditional on perceived complexity; a "simple" task carries the same requirements as a multi-file feature.
 13. **Merge authority (coordinated work):** when a coordinator/orchestrator session is active for the work, the post-review merge go-ahead is the coordinator's to give — once the required review gates pass, merge on the coordinator's confirmation; do not wait on the human owner personally. Solo (uncoordinated) delivery keeps the default: merge per gates 2 and 9. A "No self-merge" note on a PR means no UNREVIEWED self-merge — it does not suspend coordinator-authorized merges.
 14. Never hardcode secrets; never emit credential values in any output (not even partially, not "to confirm").
-15. Trunk-based git only: branch from `main`, merge via a reviewed PR (squash), never push directly to `main`.
+15. Trunk-based git only: branch from the integration trunk, merge via a reviewed PR (squash), never push directly to the trunk.
 16. If you modify source code, an independent review (author ≠ reviewer) must pass before completion.
 
 ## Integrity (quality gates are never bypassed)