docs: reframe ARK as a concept-complete reference/idea repo

[mikim]

Repositions ARK honestly as a reference implementation / idea repo (concept-complete) rather than a product in progress, and adds a runnable example so the concept is verifiable from a checkout. No package behavior changes.

① Honest framing (README)

• New top status callout: reference implementation, not published to npm, product-scale work deliberately out of scope.
• Multi-phase Roadmap + Success Metrics → one ## Status & scope section: Implemented (concept-complete) / ideas it demonstrates / Out of scope.
• Quickstart no longer implies an npm release; Initial Target Users → Who might find it useful; description products → runtimes.

② Runnable example (examples/pipeline)

• Deterministic end-to-end demo sanitize → classify → policy → report, wired to pnpm demo.
• Its README "Expected output" matches the actual run byte-for-byte (verified); typechecked by the root tsconfig so it can't silently rot.

③ Stale-noise gate — honestly scoped out

• The dangling "stale-noise (time-based) gate still pending" is declared out of scope (the burst gate already demonstrates noise suppression; a time-based gate would add speculative, non-deterministic-leaning behavior).
• Aligned the earmarked-but-unused recentWindow timestamp docs in @ark/core and the schema so they no longer claim stale-noise detection exists.

④ Fixed contradictions the reframe exposed in adjacent docs

• SECURITY.md no longer asserts published @ark/* npm releases (fixes goes to main).
• CONTRIBUTING.md / ARCHITECTURE.md frame the release tooling as configured-but-dormant.

Rigor

Built with two agent passes: a pre-write codebase read (to ground every claim) and a post-write adversarial verification (docs-vs-code accuracy, framing consistency, example faithfulness). Verdict commit_after_minor_fixes; the one must-fix (SECURITY.md) and the sensible nice-to-haves are included here.

pnpm check (typecheck + lint + schema + tests) green; pnpm demo runs and matches its documented output.