docs(l0): declaration grammar moves to .mosaic/repo.json (#1216)

Amendment per operator GO (ms-grill-me design round, 2026-08-20):

- Declaration is .mosaic/repo.json under integration_trunk, not a
  byte-exact AGENTS.md line. Unknown/misspelled keys and unparsable
  files are a structural hard stop; no prose grammar to near-miss
  (Q15 answer, plan D3/D9).
- Changing an existing declaration is operator-owned, above ordinary
  PR review (SF2, plan D10).
- Seeds this repo's declaration: integration_trunk next,
  release_branch main — prevents the absent-declaration default from
  rebinding the stack to main at land time.
- Gloss references in BOOTSTRAP/CODE-REVIEW/CI-CD-PIPELINES point at
  repo.json; merge of origin/next restores pinned-image CI coverage
  for this pre-pin branch (D27 caveat).

packages/mosaic/framework/defaults/CONSTITUTION.md

@@ -21,17 +21,19 @@ 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`.
+The **integration trunk** is the branch a project declares in its `.mosaic/repo.json` under the
+key `integration_trunk`; `release_branch` names the release target when one exists (`null` for
+single-branch projects). Absent a declaration, the trunk is `main`. The declaration is policy
+data, never shell text: values must be valid local branch names 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 declaration file that fails to parse, an unknown or
+misspelled key, or an invalid value is a hard stop (`blocked`) — never a silent fallback to `main`.
+Prose that mentions branch names designates nothing; only the declaration file does. A project
+declares exactly ONE trunk. **Changing an existing declaration is operator-owned:** a trunk
+redeclaration redirects merge target and branch-protection target at once, so it requires an
+explicit operator action above ordinary PR review. 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.