fix(ci): Enforce canonical region order in stacked-PR bodies

[thecodedrift]

Formalizes the order the managed regions appear in a stacked-PR body, and makes a stack sync reformat existing PRs to that order.

A stacked-PR body carries up to three managed regions: the stack breadcrumb (<!-- stack … -->), the human description, and carried-forward legacy (<!-- PR:N --> blocks, a merged child's body accumulated into its surviving parent). Their order was never defined — spliceRegion and upsertCarriedRegion both replace-in-place when the region exists and otherwise append, so a region stayed wherever it first landed. Depending on authoring history a PR could open with the breadcrumb, the description, or the legacy first, and reconcile only ever swapped content in place, never re-sorted.

This adds canonicalizeBody as the single layout authority and fixes the order to breadcrumb → description → carried-forward (carried sorted ascending by PR number). Both writers route through it:

• Sync path (planUpdate → reconcile): a git town sync / shape-change reconcile now also normalizes layout — the one moment the region order actually matters, per the discussion that prompted this.
• Merge path (carryForward): ends with the same canonicalization, preserving the existing breadcrumb, so whichever of the two jobs writes the parent body last, the result is the same canonical string (race-safe, idempotent, convergent).

canonicalizeBody returns a body with nothing managed (no breadcrumb to place, none present, no carried regions) byte-for-byte untouched, so a plain non-stacked PR's description is never rewritten — this preserves the old spliceRegion no-op that kept reconcile from churning ordinary PRs. spliceRegion is retired (its in-place semantics were exactly why order was never enforced); its meaningful invariants moved into canonicalizeBody tests.

Reviewer notes:

• The breadcrumb → description flip is the visible behavior change; existing PRs re-sort on their next stack sync, not retroactively.
• Workflow-only change (.github/scripts/*.cjs), no published package touched, so no changeset.
• 61 node --test cases pass; both files are prettier-clean.

🤖 Generated with Claude Code

[Copilot]

Pull request overview

This PR formalizes a canonical layout for stacked-PR bodies by introducing canonicalizeBody as the single authority for ordering managed regions, and routes both sync-time reconciliation and merge-time carry-forward through it to ensure deterministic, convergent formatting.

Changes:

• Introduces canonicalizeBody to enforce breadcrumb → description → carried-forward ordering (with carried regions sorted by PR number).
• Updates sync-time body planning (planUpdate) to reflow existing out-of-order bodies into canonical order.
• Updates merge-time carry-forward (carryForward) to canonicalize the final parent body so merge and sync paths converge.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
.github/scripts/stack-breadcrumb.cjs	Adds canonicalizeBody, removes spliceRegion, and routes reconcile + carry-forward through canonical layout.
.github/scripts/stack-breadcrumb.test.cjs	Replaces spliceRegion tests with coverage for canonical ordering, idempotence, carried sorting, and sync/merge convergence.