fix(tool): include ignored files for empty grep glob discovery

[roboomp]

Repro

In a temp project with flake.nix, home-common.nix, and nested modules/common/*.nix, the original failure returns empty results for discovery calls like glob(path="*.nix"), glob(path="modules/**/*.nix"), glob(path="modules"), grep(pattern="home\\.packages", path="."), and grep(pattern="home\\.packages", path="modules"), while exact-file controls glob(path="flake.nix"), glob(path="modules/common/zsh.nix"), and grep(..., path="home-common.nix") still work. The follow-up case is reproducible without a project .gitignore when Git ignore sources such as .git/info/exclude hide the same paths; bun .wt/repro-grep-glob-no-dotgitignore.ts confirmed the branch now returns discovery matches by default and still returns empty for explicit gitignore:true controls.

Cause

GlobTool.execute and GrepTool.execute pass omitted gitignore as true into the native filesystem walkers (packages/coding-agent/src/tools/glob.ts, packages/coding-agent/src/tools/grep.ts). The walkers honor the full Git ignore stack, not only project .gitignore, so directory/glob discovery can silently produce zero matches even when exact-file paths work because exact files bypass the walker.

Fix

• Added a default-only fallback in GlobTool that retries with ignored files included when the first gitignore-aware discovery pass returns no files.
• Added the same default-only fallback in GrepTool for filesystem discovery scans that return no matches.
• Kept explicit gitignore:true strict: callers that opt into ignored-file filtering still receive empty results for ignored paths.
• Added focused regression coverage for glob and grep discovery fallback, explicit gitignore:true, and exact-file grep controls.

Verification

bun test packages/coding-agent/src/tools/__tests__/glob.test.ts packages/coding-agent/test/tools/grep-gitignore-discovery.test.ts passed with 7 tests and 30 assertions. bun .wt/repro-grep-glob-no-dotgitignore.ts passed on the branch: no project .gitignore, default discovery returns the .nix matches, and explicit gitignore:true returns empty controls. bun run fix completed successfully before commit; gh_push_branch completed its pre-publish gate and pushed d8434d1c3636. Fixes #4706

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