Add decision-complete MVP product and architecture documentation

[codeacme17]

Summary

• define canonical, implementation-independent Repository and Worktree domain language
• add the decision-complete macOS MVP PRD with FR1–FR9, the safety contract, capability matrix, non-functional requirements, and AC-01–AC-24
• document the source and packaged process model, deep module interfaces, versioned protocol, Git execution contract, refresh and operation coordination, security boundary, testing strategy, and delivery phases
• record accepted decisions for isolating unsupported Codex host integration and placing system Git behind the Repository Engine
• link all product and architecture documents from the repository README

Acceptance criteria

• CONTEXT.md contains domain language only and no implementation details
• the PRD preserves the MVP decisions from Tracking: Deliver the Codex Git macOS MVP #1 and defines 24 release-blocking scenarios
• the architecture distinguishes ordinary documented system capabilities from the unsupported Codex CDP/DOM adapter
• two accepted ADRs capture the required hard-to-reverse architecture decisions
• all README and cross-document relative links resolve
• all GitHub-facing content and examples are in English with no generated-by or co-author notices
• no runtime behavior or executable product code is introduced

Verification

• npm run format:check
• npm run lint
• npm run typecheck
• npm test — 2 test files and 2 tests passed
• npm run build
• Git diff whitespace check
• GitHub GFM rendering for README, CONTEXT, PRD, architecture, and both ADRs
• exact count checks for FR1–FR9 and AC-01–AC-24
• two-axis review: Standards 0 findings; Spec 0 findings
• PR preflight was skipped by explicit maintainer decision

Dependency state

This is the first dependency in the MVP delivery graph and has no hard dependency. Merging it unblocks #3.

The final diff is documentation-only: 1,714 added lines across six files. The only pre-existing file changed is README.md with 10 additive lines.

Known out-of-scope defects

• The initial runtime remains a partial scaffold. Missing bootstrap acceptance work, including complete contract/integration/end-to-end coverage and standalone composition, remains tracked by Bootstrap the TypeScript workspace and standalone runtime #3.

Closes #2.