docs: refresh APS plans and Windows-first contributor experience

[joshuaboys]

Summary

Documentation/plans only: 23 Markdown files, no functional or workflow changes.

• Make Windows a first-class native instruction path: PowerShell 5.1/7 installation, PATH, updates/removal, disposable quickstart, contributor checks, examples, and MCP path/argument handling. No WSL default or execution-policy relaxation.
• Add repository context and a current architecture/decision map; distinguish durable plans, generated context, optional tooling, and proposed coordination features.
• Reconcile shipped plan evidence (INSTALL-024, v0.9 roadmap/release narrative, unreleased CIB-007/008); preserve actual REL-005 parity and CIB-010 Windows/Warp gates.
• Improve README navigation, public contributor candidates, reporting guidance, agent authority boundaries, and executable example documentation.

No new task admission, issue creation, release, merge, or auto-merge is included.

Validation

Rerun at the published documentation head:

• Full Markdown lint: pass.
• APS plan lint: 50 files, no issues.
• Git diff whitespace/scope checks: pass; all changed files are Markdown.

During preparation on the same unchanged implementation:

• 237 locked Rust tests; fmt, clippy, build: pass.
• Shell suite: pass; doctor migration leg skipped because release-mode binary was not built.
• Bash/Rust parity across 16 fixtures: pass; PowerShell leg unavailable locally.
• Native Unix journey and exact disposable Unix quickstart: pass (installer journey uses mocked download transport).
• MCP: 14 tests and TypeScript check pass; explicit native APS_BIN override also passes.
• Relative reader-doc links/anchors: 47 documents checked, no unresolved links.
• Example lint: zero errors, seven historical review-age/missing-review warnings retained rather than masking them.

Windows evidence and limits

Existing main CI at fb7df5a passed both native PowerShell 7 and Windows PowerShell 5.1 release-shaped journeys:
https://github.com/eddacraft/anvil-plan-spec/actions/runs/36534868068/job/109296785596

The new PowerShell documentation snippets were reviewed against the implementation but not executed locally: this environment has neither PowerShell nor native Windows. Existing CI is not proof of those new snippets or interactive Warp behaviour. CIB-010 remains In Progress.

The docs now disclose two real Windows boundaries rather than claim blanket parity:

• Native audit validation execution currently invokes bash; Windows users can use audit --no-run and run validation directly in PowerShell.
• MCP must explicitly select native aps.exe via APS_BIN; its repository default prefers the sibling bash script.

Pre-existing CI/policy issue (not changed here)

The PR-only anvil workflow failed before this branch: its placeholder installer returned HTTP 404, followed by command-not-found failures. Its licensed gate also depends on a secret unavailable to fork PRs:
https://github.com/eddacraft/anvil-plan-spec/actions/runs/36534585885

Repairing that installation and agreeing a fork-safe licensed-gate policy are separate work. This PR does not disable, bypass, or modify any gate, and should not be described as all-green until its own checks finish.