Bootstrap librarian Stage 1 with lychee link checker

Captures the full librarian design (two-layer architecture, three-stage
pipeline, vendor-agnostic via LIBRARIAN_LLM env, propose-only / no
auto-apply) in docs/plans/librarian.md. Lands the first concrete Stage 1
piece: scripts/librarian_check_links.py, a lychee --offline wrapper
ported from ~/dev/coolrl/src/coolrl/dev/check_doc_links.py.

Also moves the librarian prompt from .claude/agents/ (Claude Code only)
to scripts/librarian-prompt.md so any CLI can load it as a system
prompt later. Fixes one stale README link the new checker caught:
docs/classic-port-notes.md → docs/archive/classic-port-notes.md.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

README.md

@@ -48,4 +48,4 @@ while not state.terminal:
 print(state.total_score(0), state.total_score(1))
 ```
 
-See [classic port notes](docs/classic-port-notes.md) for the current direction.
+See [classic port notes](docs/archive/classic-port-notes.md) for the current direction.