feat: run docs-mcp as a long-lived shared HTTP service

context-docs was previously spawned per call as a fresh stdio container,
which meant every MCP request paid full cold-start cost (embedding model
load + Chroma open) and concurrent clients raced for the same Chroma
writer. The 50+ orphan container build-up I saw during the publish audit
was the visible symptom.

This refactor runs docs-mcp as one long-lived service:

- compose: docs-mcp leaves the 'mcp' profile, gets container_name,
  restart: unless-stopped, healthcheck, and a host port (127.0.0.1:8776
  by default). Runs as the host UID/GID so bind mounts don't end up
  root-owned.
- docker image: adds mcp-proxy (0.12.0) and an entrypoint that fronts
  llms-txt-mcp's stdio as Streamable HTTP. Reads sources from a flat
  file mounted at /etc/context-kit/docs-sources.txt. Disables eager
  preindex by default; callers refresh on demand via the docs_refresh
  tool. Set CONTEXT_KIT_DOCS_PREINDEX=1 to restore eager behavior.
- bin/context-kit: 'start' brings up the docs service alongside SearXNG,
  generates the sources file from CONTEXT_KIT_DOCS_SOURCES, and waits
  for the HTTP endpoint to become ready (up to 180s for first-run model
  download). 'docs' still works for stdio-only clients but is now a
  thin mcp-proxy bridge onto the shared HTTP service. 'doctor' and
  'status' both surface the new endpoint.
- install snippets: context-docs is now 'type: remote'/'type: http'
  pointing at ${CONTEXT_KIT_DOCS_HTTP_URL}. HTTP-capable MCP clients
  bypass the bridge entirely. snippets/*.json and the install command
  output stay byte-identical.
- docs and .env.example updated for new vars (CONTEXT_KIT_DOCS_PORT,
  CONTEXT_KIT_DOCS_HTTP_URL, CONTEXT_KIT_DOCS_PREINDEX) and the new
  24h TTL default (down from 7d; the long-lived service makes shorter
  defaults cheap).

Verified end-to-end:
- compose config -q, bash -n, sh -n all clean
- HTTP /status returns 200
- stdio bridge returns initialize + tools/list with the same 3 tools
  (docs_sources, docs_refresh, docs_query)
- doctor passes all 10 checks including the new HTTP probe
- web-search and repomix MCP handshakes still work
- redaction-check clean
- install JSON valid for both targets + --absolute

docs/configuration.md

@@ -14,29 +14,34 @@ shell code.
 | `CONTEXT_KIT_DATA_DIR` | `$HOME/.local/share/context-kit` | Persistent docs indexes and model cache |
 | `CONTEXT_KIT_COMPOSE_PROJECT` | `context-kit` | Docker Compose project and network prefix |
 | `CONTEXT_KIT_SEARXNG_PORT` | `8099` | Localhost SearXNG port |
-| `CONTEXT_KIT_DOCS_TTL` | `7d` | Docs re-fetch cadence |
+| `CONTEXT_KIT_DOCS_PORT` | `8776` | Localhost port for the long-lived docs-mcp HTTP service |
+| `CONTEXT_KIT_DOCS_HTTP_URL` | `http://127.0.0.1:${CONTEXT_KIT_DOCS_PORT}/mcp` | URL emitted into install snippets and used by the stdio bridge |
+| `CONTEXT_KIT_DOCS_TTL` | `24h` | Docs re-fetch cadence |
 | `CONTEXT_KIT_DOCS_SOURCES` | `config/sources.default.txt` | Space-separated source profile files |
 | `CONTEXT_KIT_DOCS_MAX_GET_BYTES` | `75000` | Max bytes returned by docs retrieval |
 | `CONTEXT_KIT_DOCS_EMBED_MODEL` | `BAAI/bge-small-en-v1.5` | SentenceTransformers embedding model |
+| `CONTEXT_KIT_DOCS_PREINDEX` | `0` | Set to `1` to re-embed every source on container start |
 
 ## TTL Guidance
 
-`7d` is the default because most reference docs do not need daily re-embedding.
+`24h` is the default. Most reference docs do not need re-embedding more often,
+and the shared service does not re-fetch sources until the TTL elapses.
 
 Use shorter TTLs for fast-moving APIs:
 
 ```sh
-CONTEXT_KIT_DOCS_TTL=72h bin/context-kit docs
+CONTEXT_KIT_DOCS_TTL=6h bin/context-kit restart
 ```
 
 Use longer TTLs for stable specs:
 
 ```sh
-CONTEXT_KIT_DOCS_TTL=30d bin/context-kit docs
+CONTEXT_KIT_DOCS_TTL=30d bin/context-kit restart
 ```
 
-When freshness matters for one task, prefer a manual refresh through the docs
-MCP tool instead of lowering the global TTL for every session.
+The docs-mcp container reads `CONTEXT_KIT_DOCS_TTL` at startup, so changes
+require `bin/context-kit restart`. When freshness matters for one task, prefer
+calling the `docs_refresh` MCP tool instead of lowering the global TTL.
 
 ## Source Profiles