docs: prune stale planning docs and lean out agent guide

- Remove FEATURE_SPEC.md, TODO.md, and docs/AGENT_INTEGRATION.md; their
  core gaps have shipped. Open work moves to a short ROADMAP.md.
- Remove the CLAUDE.md symlink and rewrite AGENTS.md as a quick guide:
  navigation, rules, and the nix flake check gate.
- Merge OWNERSHIP_SELF_HOSTED.md into OWNERSHIP_PUBLISH.md and drop
  private host integration notes.
- ARCHITECTURE: add MSN endpoints, clarify Bing ingest is unsupported.
- OWNERSHIP_SYNC: note source.pdf_url also carries XLSX URLs.
- systemd example: --history 2 so snapshots stay under the 10 MiB
  client cap.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

.codex/skills/idx-release/SKILL.md

@@ -16,7 +16,7 @@ ownership snapshot artifacts as a separate release track.
 Read these files before mutating release state:
 
 - `Cargo.toml` for the package version
-- `TODO.md` for the latest smoke findings and release-readiness notes
+- `ROADMAP.md` for known limits that may belong in release notes
 - `README.md` if install or publish notes may need updates
 - `docs/OWNERSHIP_SYNC.md` and `docs/OWNERSHIP_PUBLISH.md` only when checking
   ownership-sync behavior or keeping snapshot and app releases separated
@@ -59,13 +59,11 @@ curl -fsSL https://crates.io/api/v1/crates/idx-cli
 
 For a real release or release-hygiene fix, verify the repo state first.
 
-Run the standard checks inside `nix develop`:
+Run the canonical gate (fmt, clippy, tests, release build, `cargo package`
+verification, and mock smoke):
 
 ```bash
-nix develop --command cargo build
-nix develop --command cargo clippy -- -D warnings
-nix develop --command cargo test
-nix develop --command cargo package --locked
+nix flake check -L
 ```
 
 Run targeted smoke when the release changes user-facing CLI behavior. For