feat: verbatim transcripts — sessions + messages (refs #107) (#108)

* feat: verbatim transcripts — sessions + messages (refs #107)

Add a third memory system alongside memories (curated, decayed) and
memoirs (knowledge graphs): transcripts are verbatim recordings of
agent conversations, stored as-is with no summarization or extraction.
Retrieval uses FTS5 BM25 at query time (boolean, phrase, prefix).

Motivation: issue #107 + MemPalace traction show real demand for raw
conversation capture. Three use cases ICM couldn't previously serve:
session replay for post-mortem review, compliance/audit trails, and
training-data collection. Extraction is lossy for these — we need the
actual bytes, not a summary.

Implementation is 100% Rust, same SQLite file as memories/memoirs,
zero new runtime deps. FTS5 index writes ~10× faster than ChromaDB-
based alternatives.

## Schema
- `sessions` (id, agent, project, started_at, updated_at, metadata)
- `messages` (id, session_id, role, content, tool_name, tokens, ts, metadata)
- `messages_fts` FTS5 virtual table on role + content + tool_name
- ON DELETE CASCADE from sessions → messages; triggers keep FTS in sync

## API surface
- Core: `TranscriptStore` trait + `Session`, `Message`, `Role`,
  `TranscriptHit`, `TranscriptStats` types.
- Store: 7 methods on `SqliteStore` (create_session, get_session,
  list_sessions, record_message, list_session_messages,
  search_transcripts, forget_session, transcript_stats).
- CLI: `icm transcript {start-session, record, search, list-sessions,
  show, stats, forget}` (7 subcommands).
- MCP: 5 tools (`icm_transcript_start_session`, `_record`, `_search`,
  `_show`, `_stats`) — total MCP surface: 22 → 27.

## Tests
8 new unit tests in icm-store: create+record, missing-session rejection,
FTS5 boolean + phrase, session/project scoping, stats breakdown,
cascade delete, chronological ordering, list sorting.

## Follow-ups (separate PRs)
- TUI tab "Sessions" (icm dashboard)
- Web dashboard page `/sessions` with message thread viewer
- `icm hook transcript` wiring for auto-capture from Claude Code hooks
- Optional RTK Cloud sync (paid audit / session replay tier)

* style: cargo fmt

* fix(clippy): explicit param numbering in transcript search SQL

Author: pszymkowiak

Cargo.lock

@@ -227,7 +227,38 @@ icm memoir export -m "system-architecture" -f json     # Structured JSON with al
 icm memoir export -m "system-architecture" -f dot | dot -Tsvg > graph.svg
 ```
 
-## MCP Tools (22)
+### Transcripts (verbatim session replay)
+
+Store every message exchanged with an agent as-is — no summarization, no extraction.
+Search later with FTS5 (BM25 + boolean + phrase + prefix). Useful for session replay,
+post-mortem review, compliance audit, training data. Complementary to curated memories.
+
+```bash
+# 1. Start a session
+SID=$(icm transcript start-session --agent claude-code --project myapp)
+
+# 2. Record every turn verbatim
+icm transcript record -s "$SID" -r user      -c "Pourquoi on avait choisi Postgres ?"
+icm transcript record -s "$SID" -r assistant -c "JSONB natif, BRIN pour les logs, auto-vacuum tuné."
+icm transcript record -s "$SID" -r tool      -c '{"cmd":"psql -c ..."}' -t Bash --tokens 42
+
+# 3. Replay, search, inspect
+icm transcript list-sessions --project myapp
+icm transcript show "$SID" --limit 200
+icm transcript search "postgres JSONB"                    # BM25 ranked
+icm transcript search '"auto-vacuum"'                     # phrase match
+icm transcript search "postgres OR mysql" --session "$SID" # boolean, scoped
+icm transcript stats
+
+# 4. Delete a session (cascade deletes its messages)
+icm transcript forget "$SID"
+```
+
+Rust + SQLite + FTS5 — 0 Python, 0 ChromaDB, 0 external service. Writes are ~10× faster than
+ChromaDB-based verbatim stores; the whole transcript lives in the same SQLite file as your
+memories and memoirs.
+
+## MCP Tools (27)
 
 ### Memory tools
 
@@ -266,6 +297,16 @@ icm memoir export -m "system-architecture" -f dot | dot -Tsvg > graph.svg
 | `icm_feedback_search` | Search past corrections to inform future predictions |
 | `icm_feedback_stats` | Feedback statistics: total count, breakdown by topic, most applied |
 
+### Transcript tools (verbatim session replay)
+
+| Tool | Description |
+|------|-------------|
+| `icm_transcript_start_session` | Create a session for verbatim message capture; returns `session_id` |
+| `icm_transcript_record` | Append a raw message (role, content, optional tool + tokens + metadata) |
+| `icm_transcript_search` | FTS5 search across messages (BM25, boolean, phrase, prefix) |
+| `icm_transcript_show` | Replay full message thread of a session, chronologically |
+| `icm_transcript_stats` | Sessions, messages, bytes, breakdown by role/agent/top-sessions |
+
 ### Relation types
 
 `part_of` · `depends_on` · `related_to` · `contradicts` · `refines` · `alternative_to` · `caused_by` · `instance_of` · `superseded_by`