feat(tool): make repo_search portable

[AetherAI3]

Summary

• Replace shell-based repo_search with a Node-native literal search that works without grep/head.
• Keep searches inside the workspace, skip .git/node_modules/dist, ignore symlinks and binary files, and preserve the existing 40-hit cap/output envelope.
• Harden run_shell spawn-error reporting and stabilize a heartbeat timing test exposed by the Windows sandbox.

AA-LOOP-03 audit artifact

• Shortlist considered: portable repo_search, /doctor --json, MCP config export, and slash-command help export. Chosen: portable repo_search, because it fixes a real cross-platform tool gap without new privilege or dependencies.
• Concept mutation: narrowed from a new user command to a backend tool reliability fix; kept literal substring semantics so model behavior does not change unexpectedly.
• Adversarial review: PASS. Risks checked: workspace escape, symlink traversal, binary noise, dependency drift, output floods, and shell portability.
• Build/review/simplify: UI unchanged by design; backend change lives in ToolExecutor; tests cover recursive hits, ignored dirs/binary files, and cap behavior. File hygiene reviewed; implementation file is 189 lines and touched oversized test suites were already shared fixtures.

Tests

• npm run build
• node --test --test-isolation=none "dist/test/**/*.test.js" (470 pass)

Refs #36.