channel-memory: add SQLite source-message adapter

[mostlydev]

Motivation

PR #262 defines digest-backed channel awareness for issue #232 Phase 2. PR #264 landed the stable source-handle prerequisite. The next executable slice is a durable channel-message substrate; the current claw-wall store is an in-process retention window and cannot be the source of truth for digest provenance.

Scope

• Add an examples/channel-memory adapter with a service descriptor and README.
• Use SQLite as the primary store for channel source messages and derived recall blocks.
• Store source_messages keyed by (source_kind, channel_id, message_id, content_hash) so edited content creates a new source row while serving can select the current non-deleted version.
• Add tables for derived blocks, derived-block/source-message provenance, coverage gaps, and processor queue state.
• Implement idempotent ingest into the adapter and deterministic processing that works without an LLM key.
• Preserve hard events verbatim or near-verbatim; only mark rollups as sparse when they summarize or omit source content.
• Include edit/delete/tombstone and forget/visibility semantics in the data model, even if the first caller only exercises normal message ingest.

Out of scope

• Wiring claw-wall to push live Discord messages into the adapter.
• Serving raw_window+digest from claw-wall.
• Async LLM digest generation and provider cost caps.
• Release pin, changelog, or nav changes.

Acceptance

• Unit tests cover idempotent ingest, content-hash edit identity, delete/tombstone state, source-message lookup, derived-block provenance, and coverage-gap records.
• Deterministic processor tests prove useful output with no provider credentials.
• The adapter can answer exact source-message queries for retained messages by channel and message id.
• Public docs and examples stay deployment-agnostic.

Refs #262
Refs #263