feat(stack): add configurable readiness modes

[gabelease]

Adds opt-in, provider-neutral readiness policy to stacked PR/MR workflows while preserving current behavior by default.

This PR is scoped only to readiness behavior. The independent Apple Git replay compatibility fix is isolated in #40.

Why

Today every newly created change is ready unless an operator manages draft state separately. On active stacks, that can trigger review and CI repeatedly as lower changes move.

• Existing users need a no-surprises default.
• Teams need a durable root-ready policy, not one-time host commands.
• Converting an existing stack must be scoped, previewable, and reversible.

Key changes

Readiness policy

• Adds unmanaged, all-ready, and root-ready modes.
• Reads the default from git config stack.readinessMode.
• Adds one-command --readiness-mode overrides to sync and merge.
• Keeps unmanaged as the default, including host-default readiness for new changes.

Host behavior and safety

• Implements draft/ready creation and transitions for GitHub and GitLab.
• Uses sync as the explicit conversion path for existing stacks.
• Applies readiness only after branch repair, pushes, retargeting, and description updates.
• Refuses to promote a draft current root immediately before merge; operators sync it first and wait for resulting requirements.
• Journals prior readiness so undo --apply restores surviving changes safely.

Operational guidance

• Documents that linked worktrees share stack metadata, the undo journal, and repo-local config.
• Requires branch-scoped mutations for independently owned worktree stacks.
• Adds a minor changeset and updates the packaged skill.

Risks and review focus

Risk: changing readiness can trigger code-host automation.
→ Promotions occur only after repair is complete; default unmanaged performs no readiness transitions.

Risk: undo state is shared across linked worktrees.
→ Documentation makes ownership and single-journal behavior explicit, and scoped tests prove independent stacks are not touched.

Review focus: readiness journaling, provider transitions, and post-merge baseline behavior in Stack.

Test plan

bun run typecheck
PATH=/opt/homebrew/bin:$PATH bun run test
bun run lint
bun run format:check
bun run package:smoke
bun run changeset:status --since upstream/main
bun src/cli.ts sync --help
bun src/cli.ts merge --help

All 146 tests pass with Git 2.50.1. On Apple Git 2.39.5, the same seven real-Git tests fail on both upstream main and this branch because cherry-pick --empty=drop is unsupported; #40 isolates that compatibility fix.

Rollback

Revert the readiness feature and release commits. Existing installations remain behaviorally unchanged unless a managed readiness mode is configured or passed explicitly.

[hungmuha]

LGTM — well-tested, type-safe readiness modes with sound undo/journal integrity and safe defaults. One release-hygiene nit below.

[hungmuha]

nit: Changeset is marked patch, but this adds a user-facing feature (the stack.readinessMode config key plus --readiness-mode flags on sync and merge). Prior releases bumped analogous additions as minor — e.g. CHANGELOG 0.3.0 stack.blockLink/stack.trunks and 0.4.0 stack skill are all Minor Changes. Consider minor to match convention.

[gabelease]

Ah totally right. Will fix.