refactor(bootstrap): adopt sibling-only workspace layout

[xinyiklin]

Summary

Adopt a sibling-only workspace model: machine-bootstrap owns machine setup and reusable starter sources, while every project remains an independent Git repository with self-contained guidance.

This branch also adds:

• a canonical end-to-end Git/GitHub workflow and project PR template;
• audited project initialization with missing-only seeding, effective .gitignore policy checks, no-follow source/destination access, pinned identities, no-clobber writes, recoverable rollback, and hostile-concurrency regressions;
• a bounded continuity model and machine-local MACHINE.md registry;
• Product Delivery 1.3.0 role contracts and deterministic Claude/Codex adapters;
• full bootstrap CI on Ubuntu and Windows.

Scope

• Bootstrap scripts and safety helpers
• Root and project starter guidance
• Git/GitHub workflow documentation and PR templates
• Product Delivery workflow sources/adapters
• README, INIT, continuity, and cross-platform regression coverage

Legacy workspace-root AGENTS.md, CLAUDE.md, and _templates/ require deliberate manual migration; automatic migration remains intentionally unsupported.

Verification

Exact reviewed head: 867ac40214115267870329c68e2b01df46cf3a23

• GitHub Actions run 31900549095: Ubuntu and Windows jobs passed on the exact head.
• Isolated-home node scripts/test-bootstrap.mjs: all executable tests passed; 5 installed-roster checks skipped because the isolated home intentionally contains no shared skills. Python tomllib parsed 2 adversarial TOML fixtures.
• Real-home node scripts/test-bootstrap.mjs: every branch-owned check passed; only the pre-existing Impeccable install drift remains (actual 1a05c01…, reviewed manifest c882be32…).
• node scripts/init-workspace.mjs --check --skip-skills: passed; Product Delivery sources and generated provider adapters are current.
• git diff --check and JavaScript syntax checks: passed.
• Fresh independent exact-head review: no discrete correctness issues identified.
• Comments, submitted reviews, and unresolved review threads: none at final gate.
• Visual, live-app, release, and deployment checks: not applicable.

Risk and follow-up

• Reconcile the machine-local Impeccable content drift separately; this branch does not change skills.json or silently accept a new pin.
• Reviewed workflow replacement intentionally retained timestamped backups for manual recovery.
• No release or deployment is included.

Publication

• Base branch: main
• Merge strategy: squash
• Release / deploy impact: none