feat: add 6 production-ready plugins

All plugins are stdlib-only Python with no external dependencies.

- auto-summarizer: extractive conversation summary for long context
  compression, persists per-agent summaries to disk
- conversation-logger: JSONL audit logs per agent with ISO 8601
  timestamps, auto-creates log directory tree
- guardrails: safety filter detecting PII (email, phone, SSN, CC),
  prompt injection patterns, and credential exposure via regex
- keyword-memory: extracts entities (emails, URLs, dates, technical
  terms like camelCase/snake_case/dotted identifiers) as memories
- sentiment-tracker: keyword-based sentiment scoring with intensifiers
  and negation handling, only injects context for non-neutral sentiment
- todo-tracker: detects action items via 7 task patterns, tracks
  completion, deduplicates, persists per-agent with 20-item FIFO limit

plugins/conversation-logger/README.md

@@ -0,0 +1,34 @@
+# conversation-logger
+
+Logs all conversations to JSONL files for auditing, analytics, and debugging. Each agent gets its own log file at `~/.librefang/logs/conversations/{agent_id}.jsonl`.
+
+## Log Format
+
+Each line is a JSON object:
+
+```json
+{
+  "timestamp": "2026-03-21T12:34:56Z",
+  "agent_id": "agent-abc123",
+  "turn_number": 5,
+  "message_count": 10,
+  "last_user_message": "truncated to 200 chars...",
+  "last_assistant_message": "truncated to 200 chars..."
+}
+```
+
+## Hooks
+
+| Hook | Script | Description |
+|------|--------|-------------|
+| after_turn | `hooks/after_turn.py` | Appends a log entry after each conversation turn |
+
+## How It Works
+
+After each conversation turn the hook extracts summary information from the messages array and appends a single JSON line to the agent's log file. User and assistant messages are truncated to 200 characters to keep log files manageable.
+
+Errors from the filesystem (permissions, disk full, etc.) are caught silently so the agent conversation is never interrupted by a logging failure.
+
+## Usage
+
+Installed automatically when enabled in agent configuration. Log files are created on first write -- no manual setup required.