docs(adr): triage, rewrite, and renumber ADRs for clean slate

Complete ADR refactoring for issue #15:

## Changes

1. **Triage archival** — deleted chunk-era ADRs (0001–0003, 0006–0011, 0013); kept active decisions (0004, 0005, 0012, 0014+)
2. **ADR-0004 rewrite** — now reflects plugin-based skill distribution (`plugins/<name>/skills/` + `claude plugin install`) instead of monolithic `.agents/skills/` deployment
3. **Renumber to 0001–0009** — sequential clean slate after archival; all cross-references updated
4. **Content audit** — verified all 9 remaining ADRs for alignment with plugin model, removed stale chunk/deployment language

Kept ADRs: 0001–0009
- 0001: Skills distributed via plugins
- 0002: Two-tier CLAUDE.md (always-on + on-demand)
- 0003: AGENTS.md as provider-agnostic entry point
- 0004: INFO finding level in skill-audit
- 0005: agent-author dual provider scaffold
- 0006: Plugin version parity (version in both manifests)
- 0007: Gitea as exclusive issue tracker
- 0008: agent-audit single-file invocation
- 0009: agent-audit field inventory reference

All decisions are active and aligned with current repository state (marketplace/plugin model).

Closes #15 (ADR section of acceptance criteria)
Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>

docs/adr/0004-skill-audit-info-finding-level.md

@@ -0,0 +1,32 @@
+# Add INFO as a third finding level in skill-audit reports
+
+`skill-audit` shipped with two finding levels: FAIL (blocks shipping) and
+SUGGESTION (optional improvement). Provenance validation introduced observations
+that are worth surfacing but not actionable: a `references/*.md` file with no
+`source_keys` when `sources.md` is present, and a skill-level source slug absent
+from upstream research docs. Folding these into SUGGESTION would imply they
+should be fixed — but retroactive source backfill after a reference file is
+written is unreliable and not expected practice. A third level, INFO, is therefore
+introduced: observational, no action implied, never changes the pass/fail verdict.
+Counted separately in the result block as `· P info`.
+
+## Considered options
+
+**SUGGESTION with softer language (rejected)** — describe the finding as "worth
+noting" rather than "should be fixed." Rejected because SUGGESTION already carries
+an established meaning in the report; softening the language creates ambiguity
+without changing the semantic level. Downstream consumers (humans, skill-improve)
+would need to infer intent from prose rather than a stable token.
+
+**Suppress entirely (rejected)** — omit findings that have no fix. Rejected
+because the observations are useful for a human reviewing provenance completeness.
+Silent omission loses information without reducing noise.
+
+## Consequences
+
+- Report format gains a third token: FAIL, SUGGESTION, INFO. INFO findings do not
+  affect pass/fail; counted as `· P info` in the result block.
+- `skill-improve` currently ignores anything below FAIL — that behavior remains
+  correct; INFO findings are not forwarded to it.
+- Future soft observations should use INFO rather than SUGGESTION when no fix is
+  actionable.