Epic: Cross-provider contact merging & de-duplication (with settings)

[joestump-agent]

Context / Problem

msgbrowse ingests contacts from multiple providers (Signal, iMessage, WhatsApp, and later Telegram). Today contacts and their identifiers live in the contacts + contact_identifiers tables (internal/store/schema.go; get-or-create linking in internal/store/store.go ~lines 180–194; per-source delete/cleanup in internal/store/sources.go). Linking across sources is implicit and there is no user-facing control over how the same real person's identities from different providers are merged or kept separate.

This epic introduces user-controlled cross-provider contact merging and de-duplication, backed by a pluggable address-book / contact-resolver abstraction so platform-specific integrations (macOS Contacts) can be swapped for a no-op on Linux.

Goals

• Settings that control how contacts are merged across all providers (identifier matching rules, manual merge/split, overrides).
• On iOS/macOS, reference the native Contacts app to help map identifiers to real people and de-duplicate. This is platform-specific and must not be a hard dependency elsewhere.
• Because the Contacts integration is Mac-only, a pluggable/adaptable interface (ContactResolver / address-book provider) so Linux (and other platforms) supply a no-op or alternative implementation.

Design direction

• Mirror the existing interface-injection seams in internal/web: SetDetector / SetEnabler / SetPairingSource with their Enabler / PairingSource interfaces (internal/web/enable.go, internal/web/settings.go), and the detector patterns in internal/setup. The desktop shell wires the real (macOS) implementation; tests and Linux wire fakes / no-ops.
• Persistence builds on contacts + contact_identifiers; a merge likely needs a stable "canonical person" grouping plus manual override records that survive re-ingest.
• Suggest a paired spec under docs/openspec/specs/contact-merge/ (spec.md + design.md) and an ADR (MADR) under docs/adr/ (next number is 0022). Related prior art: ADR-0011 (contact-facts-extraction), docs/openspec/specs/contact-facts.

Child issues

• Pluggable ContactResolver / address-book interface + Linux no-op impl
• macOS Contacts provider implementation
• Cross-provider merge / de-dup engine (identifier matching, manual merge/split, persistence)
• Settings UI for merge rules and manual overrides
• ADR: contact-merging architecture & platform abstraction

(Child issues are also attached as GitHub sub-issues; the checklist above mirrors them.)

Relevant code

• internal/store/schema.go (contacts, contact_identifiers), internal/store/store.go (get-or-create linking), internal/store/sources.go (per-source cleanup/merge)
• internal/web/enable.go, internal/web/settings.go (seam patterns: Enabler, PairingSource, SetEnabler, SetPairingSource)
• internal/setup/ (detector patterns)
• internal/facts/ and ADR-0011 (adjacent contact work)