fix: honor --repo pointing at a module's own .code-review-graph dir

[Khusan-Khujakeldiev]

Fixes #697.

Problem

search/query/impact (and the rest of _GRAPH_TOOL_COMMANDS) resolve an
explicit --repo via find_project_root(), which only walks up looking for
.git/.svn. In a monorepo with a single .git at the root and independently
build/register-ed module subdirectories (each carrying its own
.code-review-graph marker but no .git of their own — e.g. llvm-project's
llvm/ and clang/), this silently climbs past the module's own graph to the
monorepo root and reports "No graph found" even though the graph exists
exactly where --repo pointed.

Fix

When --repo points directly at a directory that already has its own
.code-review-graph subdirectory, honor that path as-is instead of using it
only as a starting point for the upward git-root walk. This mirrors how
build/register/status already treat .code-review-graph as sufficient
evidence of a project root via tools._common._validate_repo_root.

When --repo has no .code-review-graph of its own, behavior is unchanged
(falls through to the existing find_project_root walk), so the existing
"nested subdirectory of a single repo" case is unaffected.

Testing

• Added test_tool_command_honors_repo_flag_over_ancestor_git_in_monorepo,
  which reproduces the monorepo shape (one .git at the top, a module
  subdirectory with its own .code-review-graph/graph.db) and asserts
  search --repo <module> resolves to <module>, not the ancestor with
  .git. Verified it fails on main and passes with this fix.
• Ran the full test suite locally on Windows before and after the change;
  no new failures (only pre-existing, unrelated Windows-only failures around
  chmod/symlink privileges and git submodule fixtures remain, present on
  main too).
• Verified against the real-world case that surfaced this: a 21-module,
  ~180K-file llvm-project checkout built/registered module-by-module with
  CRG, where search/query callers_of/impact --files now return correct
  results with --repo pointing at a module (e.g. the largest module,
  llvm/, ~80K files).