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/BOOTSTRAP.md

@@ -12,7 +12,7 @@ This guide covers how to bootstrap a project so AI agents (Claude, Codex, etc.)
 4. Issue tracking is consistent across projects
 5. Documentation standards and API contracts are enforced from day one
 6. PRD requirements are established before coding begins
-7. Branching/merging is consistent: `branch -> main` via PR with squash-only merges
+7. Branching/merging is consistent: branch -> integration trunk (default `main`) via PR with squash-only merges
 8. Steered-autonomy execution is enabled so agents can run end-to-end with escalation-only human intervention
 
 ## Agent Host Prerequisites
@@ -206,7 +206,7 @@ Every runtime context file should contain:
 6. **Issue tracking** — Issue and commit conventions
 7. **Code review** — Required review process
 8. **Runtime notes** — Runtime-specific behavior references
-9. **Branch and merge policy** — Trunk workflow (`branch -> main` via PR, squash-only)
+9. **Branch and merge policy** — Trunk workflow (branch -> integration trunk via PR, squash-only)
 10. **Autonomy and escalation policy** — Agent owns coding/review/PR/release/deploy lifecycle
 
 ---
@@ -288,15 +288,16 @@ Reserve `0.1.0` for the MVP release milestone.
 
 ---
 
-## Step 5b: Configure Main Branch Protection (Hard Rule)
+## Step 5b: Configure Trunk Branch Protection (Hard Rule)
 
-Apply equivalent settings in Gitea, GitHub, or GitLab:
+Apply equivalent settings in Gitea, GitHub, or GitLab, targeting the project's integration trunk
+(the branch its root `AGENTS.md` declares; default `main` — see `CONSTITUTION.md` Hard Gates):
 
-1. Protect `main` from direct pushes.
-2. Require pull requests to merge into `main`.
+1. Protect the integration trunk from direct pushes.
+2. Require pull requests to merge into the integration trunk.
 3. Require required CI/status checks to pass before merge.
 4. Require code review approval before merge.
-5. Allow **squash merge only** for PRs into `main` (disable merge commits and rebase merges for `main`).
+5. Allow **squash merge only** for PRs into the integration trunk (disable merge commits and rebase merges for it).
 
 This enforces one merge strategy across human and agent workflows.
 
@@ -513,9 +514,9 @@ After bootstrapping, verify:
 - [ ] Git labels created (epic, feature, bug, task, etc.)
 - [ ] Initial pre-MVP milestone created (0.0.1)
 - [ ] MVP milestone reserved for release (0.1.0)
-- [ ] `main` is protected from direct pushes
-- [ ] PRs into `main` are required
-- [ ] Merge method for `main` is squash-only
+- [ ] The integration trunk is protected from direct pushes
+- [ ] PRs into the integration trunk are required
+- [ ] Merge method for the integration trunk is squash-only
 - [ ] Quality gates run successfully
 - [ ] `.env.example` exists (if project uses env vars)
 - [ ] CI/CD pipeline configured (if using Woodpecker/GitHub Actions)