fix(mcp): search_code converts multi-word patterns to regex

[jjoos]

Problem

search_code returns 0 results when the pattern contains spaces (e.g. "slack webhook handler"). Single-word queries like "webhook" or "handler" work fine.

Root cause

handle_search_code writes the pattern to a temp file and passes it to grep -F -f (fixed-string mode). A multi-word pattern like "slack webhook handler" is treated as a literal substring — grep looks for that exact 22-character sequence on one line. This almost never matches real code.

Fix

Before invoking grep, if the pattern contains whitespace and use_regex is false:

1. Split on whitespace runs
2. Escape regex metacharacters in each word
3. Join with .* → slack.*webhook.*handler
4. Flip to regex mode (grep -E)

This matches lines containing all words in order. Single-word patterns are unaffected (no spaces → no conversion).

Changes

• src/mcp/mcp.c: 36 lines added in a new "Phase 0.5" block before grep invocation.
• tests/test_mcp.c: New search_code_multi_word test verifying a two-word query finds a matching line.

Testing

• New unit test: search_code_multi_word — searches for "HandleRequest error" in a fixture file containing func HandleRequest() error {. Verifies non-zero results.
• Manual testing against indexed Ruby monolith confirmed multi-word queries now return relevant results.