Define provider-neutral planning pull request foundations

[rochecompaan]

Summary

• add provider-neutral repository and pull request contracts with normalized status and distinct exhaustive-search, not-found, and identity errors
• add pure planning phase, workspace identity, marker, title, and body rules for every gate combination
• add the high-level planning workspace lifecycle contract and compile colocated fake adapters against the production interfaces
• keep current provider adapters, factories, pipeline wiring, run state, and CLI/runtime behavior unchanged

Validation

• node --test src/host/pull-requests.test.ts src/workflow/planning-pull-requests.test.ts src/git/planning-workspaces.test.ts — 23/23 passed
• npm run check:types — passed
• npm run build — passed
• npm run check:architecture — passed
• npm run lint — passed
• npm test — 1520/1520 passed
• git diff --check — passed

Reviews

• Final Codex full-worktree review — passed with no findings
• Final thermo-nuclear full-worktree review — passed with no findings
• Final validation-readiness review — passed with no findings at 1613e2c

Plan deviations

• extracted marker parsing into a focused private workflow module under the design's module-size contingency
• added test-only TypeScript configuration and Node declarations so fake adapters and negative pull request type invariants compile during npm test
• no dependency or lockfile changes

Closes #184

Patchmill run cost

Stage	Model	Prompt tokens	Output tokens	Estimated cost (USD)
Development environment	gpt-5.6-sol	472,042	3,288	$0.6767
Implementation	gpt-5.6-sol	26,585,275	271,268	$30.0007
Implementation	gpt-5.6-terra	16,698,090	72,765	$6.2555
Implementation subtotal		43,283,365	344,033	$36.2562
Total		43,755,407	347,321	$36.9329

Prompt tokens include uncached input, cache reads, and cache writes. Cost is an estimate based on Pi's recorded model pricing and includes parent and subagent sessions.