feat(tools): optional grep context lines (#76)

[jamiesun]

Summary

Closes #76.

grep previously returned only the matching line. To understand a hit (surrounding code, why it matched), the agent had to issue a second file_read of the whole file — extra ReACT turns and a large token spend. This adds an opt-in context parameter so a single grep can return the lines around each hit, the same ergonomics as grep -C.

Changes

• src/tools/search.zig
  • ContextBlock (start_line, lines, hit_mask), grepTextContext / grepFileContext.
  • Each hit expands to ±N lines; adjacent/overlapping windows merge into one block.
  • context clamped to [0, max_grep_context = 20] to bound memory and observation budget.
• src/agent.zig
  • GrepArgs gains optional context.
  • Shared grepObservation helper used by both execTool and the parallel worker:
    • context == 0 → existing formatGrepHits (behavior unchanged).
    • context > 0 → new formatGrepBlocks: hit lines lineno:text, context lines lineno-text, blocks separated by --, header notes ±N.
  • Updated the grep action description to document {"context":N}.
• docs: tools chapter (en + zh) documents the option.

Compatibility

Fully backward compatible — omitting context (or 0) keeps the exact previous output.

Testing

• zig build ✅
• zig build test ✅ (added search-level tests: ±N window with hit marking, adjacent-merge, far-apart split; agent-level grepObservation: plain vs context formatting)
• mdBook en/zh build ✅