add trace tool v2 implementation plan (TRACE-V2) (#259)

* add trace tool v2 implementation plan

Implements propose/active/TRACE-TOOL-V2-PROPOSE.md as a single PR:
tree output format, configurable collapse heuristic, source-relative
fan-out ranking, bidirectional traversal, min_result_nodes retry.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

* address plan review feedback: test counts, __all__, _build_tree detail

Must-fix:
- Fix test count in PR breakdown table (37 updated + 3 removed + 22 new = 59)
- Add __all__ export update to models step and file-by-file changes
- Expand _build_tree helper with detailed contract (multi-seed, collapsed
  reparenting, cross-service metadata, adjacency-map descent)

Should-address:
- Add bidirectional shared visited set advisory when nodes suppressed
- Document collapse_trivial=False vs collapse_roles interaction explicitly

Minor:
- Remove vestigial "Independent of" column from single-PR breakdown table

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

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Co-authored-by: Claude Opus 4.7 <noreply@anthropic.com>

Author: HumanBean17