Add dynamic service registration + GitHub Actions reindex example

[GoodbyePlanet]

Summary

Closes #83.

config.yaml is a static, curated list — fine for a handful of repos, but indexing hundreds this way means hundreds of hand-maintained entries. This adds a second, self-service way to tell semcode about a repo, and a ready-to-use GitHub Actions workflow built on it.

• Dynamic service registration: POST /reindex accepts github_repo/github_ref/root/exclude inline (alongside the existing service/force). Providing github_repo registers service with that repo definition — persisted in a new Qdrant collection (service_registry, see server/store/service_registry.py) rather than a file, so it's safe to write from an unauthenticated request and survives restarts.
  • Services resolve through load_effective_services() (config.yaml ∪ registry) everywhere a service name is looked up — IndexPipeline, GitHistoryPipeline, get_code_context, index_stats. config.yaml always wins on a name collision, so a stray/malicious registration can never silently repoint a curated service.
  • index_stats now lists "From config.yaml" and "Registered dynamically via API" separately.
  • No new auth was added — /reindex is unauthenticated the same way it already was for config.yaml services.
• examples/github-actions/reindex-on-merge.yml: drop-in workflow for the indexed repo's own CI. On push to main/master it self-registers (default service name = owner-repo, collision-safe across orgs) and reindexes in the same call — no central file to edit, no server restart.
• config.yaml is now genuinely optional:
  • Settings.load_services() treats a missing or empty config.yaml as zero services instead of raising FileNotFoundError.
  • If CONFIG_PATH resolves to a directory (the classic Docker bind-mount-of-a-missing-file footgun), it now raises a clear, actionable error instead of a cryptic IsADirectoryError.
  • Dockerfile no longer bakes config.yaml in at build time.
  • docker-compose.yaml no longer mounts config.yaml by default; docker-compose.config-yaml.yml is a small overlay that adds it back, wired up via new -with-config Makefile targets, for anyone who wants curated services alongside or instead of dynamic registration.
• Bug fix found while wiring this up: get_code_context only ever resolved services from config.yaml, so it would report a dynamically-registered service as "no longer in config.yaml" even while it was actively indexed. Fixed to use the same merged resolution as everything else.
• Docs updated: docs/configuration.md gets a full "Dynamic Service Registration" section; docs/README.md, docs/ingestion.md, docs/retrieval-rrf.md, and the root README.md updated to describe both paths and how they interact.

Test plan

• uv run pytest -q — 289 passed
• uvx ruff check . / uvx ruff format --check . — clean
• Verified docker compose -f docker-compose.yaml -f docker-compose.config-yaml.yml config merges the config.yaml volume mount correctly on top of the base file
• Manual smoke test against a live stack: POST /reindex with github_repo for a repo not in config.yaml, confirm it indexes and shows up in list_indexed_services/index_stats, and survives a container restart

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