fix(agent): block grep_files from leaking .env secrets

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Summary

• read_file already hard-blocks .env / .key / .pem and redacts secret-like keys in other files. grep_files used the same workspace resolver but skipped both checks.
• A normal agent call such as grep_files {"pattern":"API_KEY","path":"."} returned raw .env lines (for example API_KEY=...) into the model context. A 50-line .env could also consume the match cap and hide later non-secret hits.
• The fix fails closed on a direct sensitive target, skips those files before reading them during a directory walk, and runs remaining match lines through the existing redaction helper.

Task Linkage

• Task ID: TASK-2026-071
• Task folder: tasks/TASK-2026-071-block-grep-sensitive-files/

Injected Specs

• spec/architecture-boundaries.md (if architecture/layering changed)
• spec/security-nonnegotiables.md (if sandbox/security changed)
• spec/testing-policy.md (required for any code change)
• spec/docs-sync.md (if behavior/docs/env/commands changed)

Validation Evidence

• Commands executed:
  • python3 scripts/validate_tasks.py — passed (71 task directories)
  • cargo fmt --check — exit 0
  • cargo clippy --all-targets -- -D warnings — exit 0
  • cargo test -p skilllite-agent grep_files — 10 passed, 0 failed
  • cargo test — all workspace crates ok, 0 failed (skilllite-agent 250 passed)
• Key results:
  • test_grep_files_blocks_direct_sensitive_path passed
  • test_grep_files_skips_dotenv_during_workspace_walk passed
  • test_grep_files_redacts_sensitive_keys_in_normal_files passed

Regression Scope

• Areas likely affected:
  • Agent grep_files builtin
  • skilllite-fs::grep_directory optional skip-file callback
• Explicit non-goals:
  • Dotenv variant suffix hardening (#143)
  • Recovered write_file clobber (#144 / #146)
  • Symlink canonicalize (#132)
  • Preview-server symlink follow
  • ChatSession workspace-root alignment (#114)

Docs Sync (EN/ZH)

• Updated EN + ZH docs
• Files:
  • docs/en/ARCHITECTURE.md
  • docs/zh/ARCHITECTURE.md

Review Checklist

• Acceptance criteria in tasks/TASK-2026-071-block-grep-sensitive-files/TASK.md satisfied (or explicitly deferred)
• tasks/TASK-2026-071-block-grep-sensitive-files/STATUS.md updated with latest progress
• tasks/TASK-2026-071-block-grep-sensitive-files/REVIEW.md includes merge readiness decision
• tasks/board.md status is up to date