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/auto-summarizer/README.md

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+# auto-summarizer
+
+Maintains a running conversation summary to help agents handle long conversations without losing context. Uses extractive summarization (no ML or external dependencies) to identify the most important parts of a conversation.
+
+## How it works
+
+After each conversation turn, the plugin scans all messages and extracts:
+
+- **Topic opener** -- the first user message that started the conversation
+- **Questions** -- any messages containing questions (detected via `?`)
+- **Decisions** -- messages with conclusion/decision language ("let's", "decided", "the plan is", etc.)
+- **Recent context** -- the last 2 exchanges to preserve immediate context
+
+These are combined into a compact summary (max 500 characters) and persisted to disk. On the next ingest, the summary is returned as a memory fragment so the agent retains awareness of the full conversation.
+
+Summarization only activates when the conversation exceeds 6 messages -- shorter conversations are passed through as-is.
+
+## Hooks
+
+| Hook | Script | Description |
+|------|--------|-------------|
+| ingest | `hooks/ingest.py` | Returns the stored conversation summary as a memory fragment |
+| after_turn | `hooks/after_turn.py` | Builds and persists an extractive summary of the conversation |
+
+## Storage
+
+Summaries are stored at `~/.librefang/plugins/auto-summarizer/{agent_id}.summary`.
+
+## Usage
+
+Installed automatically when enabled in agent configuration.