feat: add /healthz and /metrics endpoints, replace print with logging

- Add /healthz endpoint returning LLM provider validation status
- Add /metrics endpoint serving Prometheus metrics via prometheus_client
- Replace all print() calls in health.py with proper logging module
- Remove _PREFIX variable in favor of structured logger context

docs/mnemosyne_integration.md

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+# Mnemosyne Integration — Pallas Reference
+
+This document summarises the Pallas-specific changes required for Mnemosyne knowledge integration. The full specification lives in [`daedalus/docs/mnemosyne_integration.md`](../../daedalus/docs/mnemosyne_integration.md).
+
+---
+
+## Overview
+
+Pallas agents gain access to Mnemosyne's content-type-aware knowledge graph as a downstream MCP server. Agents can search documents, browse libraries, retrieve items, and traverse the concept graph — all via standard MCP tool calls.
+
+---
+
+## Configuration Changes
+
+### fastagent.config.yaml
+
+Add the Mnemosyne MCP server:
+
+```yaml
+mcp:
+  servers:
+    # ... existing servers (argos, neo4j_cypher, kernos, rommie, gitea, grafana) ...
+    mnemosyne:
+      transport: http
+      url: "http://puck.incus:22091/mcp"
+```
+
+### Agent Definitions
+
+#### Research Agent (port 23031)
+
+The `knowledge` agent in the research chain gains Mnemosyne access:
+
+```python
+@fast.agent(name="search", servers=["argos"])
+@fast.agent(name="knowledge", servers=["neo4j_cypher", "mnemosyne"])
+@fast.chain(name="research", sequence=["search", "knowledge"], default=True)
+```
+
+The `knowledge` agent's system instruction should guide tool selection:
+
+> Use `mnemosyne.search_knowledge` for document content retrieval — it handles chunking, vector search, re-ranking, and content-type-aware context. Use `neo4j_cypher` for graph topology queries, relationship exploration, and data not managed by Mnemosyne.
+
+#### Infrastructure Agent (port 23032)
+
+No changes — Infrastructure does not use Mnemosyne.
+
+#### Orchestrator (port 23033)
+
+```python
+@fast.agent(name="research_sub", servers=["argos", "neo4j_cypher", "mnemosyne"])
+@fast.agent(name="infra_sub", servers=["kernos", "gitea", "rommie"])
+@fast.orchestrator(name="orchestrator", agents=["research_sub", "infra_sub"],
+                   plan_type="iterative", default=True)
+```
+
+### Registry Update
+
+Update agent descriptions to reflect Mnemosyne access:
+
+```json
+{
+  "server": {
+    "name": "ca.helu.ouranos/pallas-research",
+    "title": "Research Agent",
+    "description": "Web search via Argos, knowledge graph via Neo4j, and content library search via Mnemosyne",
+    "version": "1.1.0",
+    "remotes": [
+      { "type": "streamable-http", "url": "http://puck.incus:23031/mcp" }
+    ]
+  }
+}
+```
+
+---
+
+## Available Mnemosyne MCP Tools
+
+These tools become available to agents with `mnemosyne` in their `servers` list:
+
+| Tool | Purpose | When to Use |
+|------|---------|-------------|
+| `search_knowledge` | Hybrid vector + full-text + graph search with re-ranking | Document content retrieval, question answering over stored knowledge |
+| `search_by_category` | Search scoped to a library type (fiction, technical, etc.) | When the user specifies or implies a content domain |
+| `list_libraries` | List all knowledge libraries | Discovering what knowledge domains exist |
+| `list_collections` | List collections within a library | Browsing a specific knowledge domain |
+| `get_item` | Retrieve item metadata + chunk previews + concept links | Deep dive on a specific document/item |
+| `get_concepts` | Traverse concept graph | Exploring relationships between topics, people, places |
+
+---
+
+## Downstream MCP Servers (Updated)
+
+| Server | Host | URL | Used by |
+|--------|------|-----|---------|
+| argos | miranda.incus | `http://miranda.incus:25534/mcp` | Research, Orchestrator |
+| neo4j_cypher | circe.helu.ca | `http://circe.helu.ca:22034/mcp` | Research, Orchestrator |
+| **mnemosyne** | **puck.incus** | **`http://puck.incus:22091/mcp`** | **Research, Orchestrator** |
+| kernos | caliban.incus | `http://caliban.incus:22021/mcp` | Infrastructure, Orchestrator |
+| gitea | miranda.incus | `http://miranda.incus:25535/mcp` | Infrastructure, Orchestrator |
+| rommie | caliban.incus | `http://caliban.incus:22031/mcp` | Infrastructure, Orchestrator |
+| grafana | miranda.incus | `http://miranda.incus:25533/mcp` | Infrastructure |