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/guardrails/README.md

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+# guardrails
+
+Safety filter plugin that detects potentially harmful content patterns in user messages and injects warning memories into agent context. Uses only Python stdlib regex -- no external dependencies.
+
+## Detection Categories
+
+| Category | Examples | Memory Tag |
+|----------|----------|------------|
+| PII | Email addresses, phone numbers, SSNs, credit card numbers | `[guardrails:pii]` |
+| Prompt injection | "ignore previous instructions", "you are now", "system prompt:" | `[guardrails:injection]` |
+| Credentials | `password=`, `api_key=`, `secret=`, `token=`, PEM private keys | `[guardrails:credential]` |
+
+## Hooks
+
+| Hook | Script | Description |
+|------|--------|-------------|
+| ingest | `hooks/ingest.py` | Scans user messages for harmful patterns and returns warning memories |
+
+## How It Works
+
+When a user message arrives, the ingest hook runs all pattern checks against it. For each detected issue a memory is returned with the category tag and a recommendation for the agent (e.g. "avoid echoing PII", "maintain original instructions"). If nothing is detected the plugin returns an empty memories list.
+
+All patterns use word boundaries and anchoring to minimise false positives on casual conversation.
+
+## Usage
+
+Installed automatically when enabled in agent configuration.