Add hierarchical .gito/config.toml support

[praphulmc-tt]

Summary

Adds hierarchical .gito/config.toml support: every <subdir>/.gito/config.toml
under the repo root is discovered and merged with bundled defaults + the root
project config.

Merge rules (each level contributes only the keys it explicitly wrote):

• Lists (aux_files, exclude_files, mention_triggers) — accumulate across
  all levels, deduped.
• prompt_vars — union of keys; root wins on key conflict.
• pipeline_steps — union of step names; root wins on name conflict.
  Per-step field-level merge is preserved (root setting enabled=false still
  inherits call from defaults).
• Scalars (prompt, retries, max_code_tokens, …) — root supersedes;
  sub-directory configs only fill values the root left unset.

Pipeline step scoping: steps defined inside a sub-directory carry a scope
field equal to that directory; Pipeline.run filters ctx.diff to files under
the scope and skips the step entirely when no diff files match. Steps defined
in the root config keep scope="" and remain repo-wide.

Back-compat: the existing single-file ProjectConfig.load(config_path=...)
API is unchanged; if no repo_root is passed, discovery is skipped and
behavior matches main.

Test plan

• 5 new tests in tests/test_project_config.py: cumulative lists, root-wins
  scalars, root-wins prompt_vars, pipeline step scope + per-field inheritance,
  ignored-dir pruning.
• 3 new tests in tests/test_pipeline.py: matches_path boundaries, scoped
  diff filtering, scoped-step skip when no matching files.
• All existing tests still pass: pytest tests/test_project_config.py tests/test_pipeline.py → 18
  passed locally.
• Ran gito review --against main against this branch locally — no issues found.