feat: in-process ast-grep structural search tool

[ScriptedAlchemy]

What

Adds tracedecay_ast_grep_search — a read-only structural (AST) search tool for call shapes, argument orders, and expression forms that a text regex cannot express. Structural search previously had no tool at all; only tracedecay_ast_grep_rewrite existed.

Backend: ast-grep as a library, not the CLI

Per the design directive, this evaluates ast-grep-core as an in-process backend and ships it:

• Compat check (passed): ast-grep-core 0.44 pins tree-sitter ^0.26.3; this repo builds its grammars against tree-sitter 0.26.8 — same 0.26 minor, ABI-compatible. ast_grep_core::tree_sitter::TSLanguage is a re-export of tree_sitter::Language, exactly what extraction::ts_provider::try_language already returns.
• Wiring: a small TdLang wrapper implements ast-grep-core's Language/LanguageExt traits over the repo's own bundled grammars — no new grammar crates, no external ast-grep binary, and no ast_grep_available() gating (the tool registers unconditionally). Metavariable pre-processing and per-language expando chars (µ, 𐀀, _) are mirrored from ast-grep-language so pattern semantics match the upstream CLI.
• The existing CLI-backed ast_grep_rewrite tool is left unchanged (library-backed rewrite, with its apply/diff semantics, is a follow-up).

New deps: ast-grep-core (+ bit-set/bit-vec). No new tree-sitter grammar crates.

Tool

• Args: pattern (required), lang (optional; auto-detected per file otherwise), path_glob (optional scope), max_results (default 50, cap 200).
• Output: markdown (default) + format: json; each hit is graph-enriched with its enclosing symbol like tracedecay_grep, so the next call is tracedecay_body. CLI twin works: tracedecay tool tracedecay_ast_grep_search.

Discovery wiring

• Hints: the search category now routes structural patterns to tracedecay_ast_grep_search (the edit category keeps its ast_grep_rewrite mention). Hint-signature drift guard stays green.
• Skills: exploring-code gains a structural-search routing row; editing-safely links search→rewrite.
• Evals: one active search scenario grounded in the fixture (reserve_stock(...) three-arg call shape in place_order), and one deferred rewrite scenario (fixture has a single call site and the rewrite tool is CLI-gated).

Gates (all green)

• cargo fmt --check
• cargo clippy --workspace --all-targets --locked -- -D warnings
• cargo nextest run --test mcp_suite --test agent_suite --no-fail-fast → 852 + 488 pass
• grade.py --lint-only (53 neutral), --check-hints (21 signatures intact), selftest.py (all pass)
• 8 in-process engine unit tests

Live smoke (against this repo)

pattern: "ToolResult::new($$$)" → real matches with enclosing symbols, e.g.:

- src/mcp/tools/handlers/edit.rs:188 — text_tool_result (function)
  > ToolResult::new( json!({ "content": [{ "type": "text", "text": text }] }), touched_files, )
- src/mcp/tools/handlers/workflow.rs:245 — handle_diagnose (function)

🤖 Generated with Claude Code

Review follow-up

The working-tree walk and AST parsing now run on Tokio's blocking pool so a broad or zero-match search cannot stall the MCP async runtime. Added a current-thread wrapper regression; focused engine tests and full clippy remain green.

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