Citation graph: fill cites / cited_by from TEI references

[deOliveira-R]

With `item_artifacts` + `extract/tei.py` landed (commit e2b7342), we have structured references for every paper with a `grobid_tei` artifact. The schema already has `cites` / `cited_by` in the `item_relations` CHECK vocabulary — they've just never been written to.

Plan

1. New CLI subcommand: `grimoire citations build [--force] [--limit N]`.
2. For each item with a `grobid_tei` artifact:
   • Parse structure via `tei.parse_structure`.
   • For each parsed reference that has a DOI:
     • Look up the DOI in our `items` table.
     • If found AND the cited item != the source item: insert `cites(source, target)` via `dedup.apply_link` (creates the symmetric `cited_by` inverse automatically — plan §7 invariant 3).
   • References whose DOIs aren't in the library go into a "suggested to ingest" list (issue Phase 7 follow-up: expose tier-4 'needs review' queue in migration report #15 review-queue-adjacent — could share the same table).
3. Idempotent via the `INSERT OR IGNORE` on the relations table.

Unlocks

• MCP `list_related(item_id, kind='citations')` already has the code path for this relation; it just returns nothing today.
• Queries like "papers in my library that cite X" and "what does paper Y cite that I have" become one SQL hop.
• Claude Code can ask "trace the citation chain forward/backward from this paper" via repeated `list_related` calls.

Acceptance

• `grimoire citations build` walks all items with a TEI artifact and writes `cites`/`cited_by` edges for DOI-matched references.
• Self-citations (same item as source and target) are skipped.
• Re-running is a no-op (`INSERT OR IGNORE`).
• Test with a synthetic TEI → seeded `items` matching the DOI → assert the edges + inverse appear.
• At least a terse status line: "N items scanned, M edges inserted, K unresolved DOIs."

Caveats

• GROBID reference-section parsing quality drops on chapter-in-edited-book style references — expect some noise.
• DOI-matching is exact; consider a normalization pass (strip `https://doi.org/\`, lowercase) to avoid missing hits.
• Bibliography entries without DOIs are genuinely unrecoverable from TEI alone — surface them in the "suggested" queue, don't try to fuzzy-match.