feat: add code-navigation tools (grep, glob, ls) + read_file line ranges (#1670)

Gives the agent first-class code navigation instead of shelling out via bash
(token-heavy, unreliable on weaker models, unstructured). Mirrors the
Grep/Glob/Read primitives that Claude Code / opencode expose.

- grep: regex search over file contents across a tree. Uses ripgrep when
  available (with explicit excludes so junk dirs are skipped even without a
  .gitignore); falls back to a pure-Python walk+regex when rg is absent.
  Returns file:line:match, capped.
- glob: find files by glob pattern (recursive), newest first.
- ls: list a directory (folders first, then files with sizes).
- read_file: optional offset/limit for line-range reads of large files
  (plain-path calls stay back-compatible).

All confined by the same path policy as read_file (_resolve_tool_path:
data/tmp allowlist + sensitive-file deny). Junk dirs (.git, node_modules,
venv, __pycache__, dist/build, …) skipped. Output capped (200 hits,
400 chars/line). Admin-gated like the other filesystem tools.

Wiring: schemas + native arg->content serializer (src/tool_schemas.py), tool
tags (src/agent_tools.py), always-available + descriptions (src/tool_index.py),
admin gate (src/tool_security.py), dispatch + impls (src/tool_execution.py).

Tests: tests/test_code_nav_tools.py — match/skip-junk/ignore-case/glob-filter,
allowlist rejection, glob/ls, read-range, and the no-ripgrep Python fallback.

src/agent_tools.py

@@ -27,6 +27,7 @@ MAX_READ_CHARS = 20_000
 
 # Tool types that trigger execution
 TOOL_TAGS = {"bash", "python", "web_search", "web_fetch", "read_file", "write_file", "edit_file",
+             "grep", "glob", "ls",
              "create_document", "update_document", "edit_document",
              "search_chats",
              "chat_with_model", "create_session", "list_sessions",