Add a contributor guide for the durable Codex lifecycle contract

[Punisheroot]

Context

The parent-owned lifecycle model and SQLite journal are implemented and tested,
but contributors currently have to reconstruct phase rules, ownership, replay,
and the boundary between implemented and pending behavior from source and
several broad documents.

Outcome

Add one concise contributor guide explaining the lifecycle domain, legal phase
flow, parent-versus-worker ownership, persistence/replay model, artifact
references, cancellation/recovery semantics, and the implementation status that
exists on main when the documentation is merged.

Scope

• Document the legal explore -> implement -> test -> review -> verify -> apply
  sequence and terminal states.
• Explain parent-owned transitions and the typed result each phase worker may
  return.
• Explain frozen profile, test-plan, source, budget, artifact, approval, and
  state-digest identities.
• Describe the SQLite event journal and deterministic replay with links to
  source and focused tests.
• Verify the status at merge time: distinguish the implemented domain/store from
  whichever executor, read API, and timeline UI slices are still pending then.
• Link the guide from existing architecture or verified-change documentation
  without copying their full content.

Non-goals

• Changing Rust or TypeScript behavior, inventing future APIs, documenting
  non-Codex execution, or presenting offline tests as provider-backed evidence.
• A tutorial that performs provider calls or modifies a real worktree.

Acceptance criteria

• Every phase, terminal outcome, and ownership statement matches current
  source and tests.
• Implemented and pending lifecycle capabilities reflect main at merge
  time, including any concurrently completed issue.
• Source links target current files or symbols and avoid fragile line
  numbers.
• Existing architecture and verified-change documents link to the guide
  without contradictory status text.
• git diff --check passes and manual review confirms every repository link
  resolves.

Starting points

• crates/needle-core/src/lifecycle.rs
• crates/needle-core/src/lifecycle/model.rs
• crates/needle-core/src/lifecycle/tests.rs
• crates/needle-runtime/src/store/lifecycles.rs
• crates/needle-runtime/src/store/lifecycles/tests.rs
• PROJECT_STATUS.md

Validation

Run git diff --check and verify each relative link against the current
checkout. No Cargo, frontend, provider, or workspace mutation is required for
this documentation-only issue.