feat(entities): gate ambiguous bare first names on corroboration (#36)

[tenfourty]

What

Stops bare common first names from auto-linking to the wrong entity.

A bare single first name (≥4 chars) that is ambiguous — claimed by 2+ entities — no longer auto-links in the participant, title, or content tiers. It links only when the entity is corroborated in the same document by a stronger signal:

• a tag (tagged) or source-ref (source_ref) match,
• a multi-word / full-name match (participant / title / discussed),
• an exact title-part match, or
• appearing in the frontmatter attendees.

Unambiguous bare first names (owned by exactly one entity) still link as before — no regression to existing single-name linking.

Accent-folding

Ambiguity is computed by the new build_first_name_index(), which accent-folds names (Jérémy → jeremy) so an accented name and its ASCII near-twin land in the same ambiguity bucket. Folding is used only to widen the ambiguity set (making the matcher more conservative) — it never broadens a match. So a bare ASCII "Jeremy" that could be either "Jérémy Cotineau" or "Jeremy Brown" is gated until one of them is corroborated. This directly addresses the issue's note that folding "should probably not, on its own, be enough to auto-link a bare first name."

Wiring

The indexer builds the name_owners index once and threads it plus the document's frontmatter attendees into find_entity_mentions.

Why

Reported by a Chief-of-Staff agent seeing common (incl. French) first names auto-matched to the wrong colleague — and, because mentions had no un-link until #35, two distinct people could end up looking like one identity.

Design note

The fix gates ambiguous bare names only — not all common names — because the existing test contract (and real usage) relies on unambiguous single names like "Soren"/"Wren" linking from content alone. Gating every bare name would mass-under-link. "Ambiguity + corroboration" is the targeted, test-consistent reading of the reported failure ("wrong colleague" / merged identity both require ≥2 candidates).

Tests

• test_entities.py::TestFirstNameDisambiguation — ambiguity index (incl. accent-fold), ambiguous bare name not linked, rescued by full-name / attendee corroboration, accent-twin gating, title gating, unambiguous still links.
• test_disambiguation.py — full index_all integration: bare ambiguous not linked; full-name and frontmatter-attendee corroboration each rescue exactly one entity (verifies the indexer threads attendees + name_owners).
• Two pre-existing tests that used the ambiguous "Anders" to assert auto-linking were updated to the new behaviour (they encoded exactly the false positive this fixes); repurposed to unambiguous names to keep their threshold/substring intent.

Docs: docs/entities.md §Disambiguation → "Bare ambiguous first names (#36)".

Closes #36.