Add channel-memory SQLite adapter example

[mostlydev]

Summary

• add a standalone examples/channel-memory HTTP service with SQLite-backed source-message retention
• key source messages by source_kind/channel_id/message_id/content_hash, with explicit observed_seq, observed_at, is_current, and superseded_by edit-version tracking
• add deterministic no-LLM digest blocks for hard_event, raw_excerpt, telemetry_count, tombstone, plus coverage-gap metadata and exact source-message lookup
• add descriptor, Dockerfile, README, and tests covering idempotent ingest, edit identity, tombstones, forget suppression, provenance, coverage gaps, and HTTP API behavior

Closes #265
Refs #262 #263

Verification

• go test -v -timeout 60s ./examples/channel-memory
• go test ./...
• go vet ./...
• git diff --cached --check
• docker build -f examples/channel-memory/Dockerfile -t clawdapus-channel-memory:test .
• docker run --rm --name clawdapus-channel-memory-test -d clawdapus-channel-memory:test + docker exec clawdapus-channel-memory-test wget -qO- http://localhost:8080/health

Notes

This intentionally does not wire claw-wall push ingest, serve raw_window+digest from claw-wall, or add async LLM/cost-cap processing. Those remain in #266, #267, and #268.