feat(mcp): default tool surface trimmed to 4 — explore, node, search, callers (#818)

callees/impact/files/status stay fully functional (handlers, CLI, library
API untouched; CODEGRAPH_MCP_TOOLS re-enables any) but are no longer
LISTED by default. Evidence: codegraph_impact appears in zero recorded
eval runs ever; its blast-radius info already arrives inline on explore
(Blast radius section) and node (dependents note). callees is redundant
by construction (a symbol's body IS its callee list). files/status
"reduce to one grep" per the tiny-repo audit, and staleness banners
already inline pending-sync. callers stays: exhaustive call-site
enumeration (incl. callback registrations, per-definition sections) is
the one job explore/node don't replicate. Fewer tools = fewer mis-picks
+ ~300 schema tokens saved per session; presence itself steers.

server-instructions rewritten around the 4-tool surface ("what does X
call" → node body+trail; "what breaks" → callers + inline blast radius).
Tiny-repo gate unchanged (its trio ⊆ the default set); stale gate
comment corrected (context/trace are long gone — its "5 core tools" are
today's trio).

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

CHANGELOG.md

@@ -16,6 +16,7 @@ and adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
 ### New Features
 
+- **The MCP tool list is now a focused default of four** — `codegraph_explore`, `codegraph_node`, `codegraph_search`, and `codegraph_callers`. The other four (`codegraph_callees`, `codegraph_impact`, `codegraph_files`, `codegraph_status`) remain fully functional — the CLI and library API are unchanged, and `CODEGRAPH_MCP_TOOLS` re-enables any of them — but they're no longer listed to agents by default: measured agent behavior shows they're never or rarely picked, and the information they carry already arrives inline on the tools agents do use (explore's blast-radius section, node's dependents note, a symbol's own body as its callee list). A leaner list saves context tokens every session and steers agents to the right tool by presence alone.
 - **CodeGraph now goes quiet instead of failing loudly in unindexed projects.** When an AI agent's session starts in a workspace that has no CodeGraph index, the MCP server now announces itself as inactive with a short note and lists no tools at all — instead of presenting the full toolset and erroring on every call, which taught agents to distrust CodeGraph even where it works. Querying another project that isn't indexed likewise returns clear guidance (use your regular tools for that codebase; the user can run `codegraph init` there to enable CodeGraph) instead of an error, and genuine internal errors now tell the agent to retry once rather than give up on CodeGraph entirely. Indexing stays your decision — agents are told not to run it themselves. (#769)
 - **Astro projects are now indexed.** `.astro` files previously weren't parsed at all — on a typical Astro site that left most of the codebase invisible to search, impact, and `codegraph_explore`. CodeGraph now extracts the TypeScript frontmatter (functions, imports, `getStaticPaths`, …) and client-side `<script>` blocks, captures function calls and `<Component>` usages in template markup so cross-component dependencies trace end-to-end, resolves the `Astro` global and `astro:*` module imports as framework-provided, and maps `src/pages/` file-based routing to route nodes (`.astro` pages and `.ts` endpoints, including `[param]` and `[...rest]` dynamic segments, with underscore-prefixed files correctly excluded). Validated on two real-world Astro sites with 93% measured cross-file coverage and every page mapping to its route. Thanks @xingwangzhe. (#768) (Astro)
 - Same-named symbols across a monorepo's apps are no longer conflated. In a NestJS-style workspace with one `UserService` per app, `codegraph_callers`, `codegraph_callees`, and `codegraph_impact` now report **one section per distinct definition** — each app's callers and blast radius under its own file-labeled heading — instead of a single merged list, and accept a `file` argument to focus exactly the definition you mean (like `codegraph_node` already did). Impact in particular no longer overstates a change's blast radius by merging unrelated same-named classes. Thanks @Igorgro. (#764)