feat(intercom): intercom resolver for contact profile and conversation metadata

[jaylann]

Summary

• New first-party package effaced-intercom (IntercomResolver) — exports a data subject's Intercom contact profile + conversation metadata (Art. 15) and erases the contact (Art. 17), mirroring the effaced-resend shape (httpx, fresh client per call, faked at the transport boundary, AttestingResolver, no rectify).
• Refs are kind="intercom", value=<Intercom internal contact id>; get/delete address the contact by id directly (no email search). Conversation metadata is paged through POST /conversations/search filtered by contact_ids.
• Wires the package into ruff/mypy/coverage, the generated API reference, PROOFS.md, and flips the roadmap entry to shipped.

Closes #61

Erasure/export semantics

New surface, additive — not a change to what gets deleted or exported for any existing resolver, so MINOR (not breaking).

• Export (Art. 15): contact email/phone (CONTACT), name (IDENTITY); per-conversation created_at/updated_at/state (BEHAVIORAL), keyed conversation.{id}.{field}. Never exported: message bodies (conversation_parts, source.body) and the caller-defined custom_attributes blob — declared as covered-surface exclusions.
• Erase (Art. 17): DELETE /contacts/{id}. A 404 on get/delete is success (already_absent=True); idempotent by construction. Erasure deletes the contact only — conversation retention is Intercom's, documented in the README and the covered-surface notes.
• The exported field set is behaviour under widened SemVer going forward: removing/recategorising a field later is MAJOR.

Risk

• Low: net-new package, zero change to existing code paths or exports. Pure-additive wiring entries.
• The exact Intercom permanent-deletion semantics (DELETE /contacts/{id} archives vs. hard-deletes) and the conversation-search contact_ids field are pinned against current Intercom docs; the resolver is robust to either via the 404-idempotency contract and records what it did in ResolverErasure.detail.
• Not added to release-please-config.json/manifest yet — matching the existing effaced-django precedent on stage (release wiring added deliberately when publishing is intended). Easy follow-up when this package is slated to publish.

Checklist

• Targets stage (never main)
• PR title is Conventional Commits (type(scope)?: lowercase subject)
• All commits DCO signed-off (git commit -s)
• just check and just test green locally
• Tests added/updated — erasure/export paths prove no cross-subject bleed (test_intercom_properties.py)
• Public API has Google-style docstrings
• Manifest format untouched, or schema version bumped + migration added
• At least one type:* and relevant area:* labels set