vedit: bisect command (find regression-introducing commit)

[explicit09]

Problem

vedit ships content-addressed linear history but has no bisect analog. When a director says "the cut felt better two days ago," there is no way to binary-search vedit history with a yes/no predicate to find the commit that regressed the feel.

Scope (MVP)

1. New tool vedit_bisect { good: <ref>, bad: <ref>, predicate: <command-or-tool> }.
2. Two interaction modes:
   • Interactive: tool returns next candidate commit, user marks good/bad, repeat.
   • Automated: predicate is another vedit tool (e.g. vedit_diff_score) that returns a verdict — bisect runs to completion automatically.
3. CLI: awidat vedit bisect start --good <ref> --bad <ref> then awidat vedit bisect good|bad.

Layers

1. crates/core/src/vc/bisect.rs (new) — bisect state machine
2. crates/core/src/tools/vedit_bisect.rs (new)
3. crates/cli/src/ — bisect subcommand
4. Test: 5-commit synthetic history, bisect converges on injected bad commit

References

• Gap analysis: .reference-research/pro-editing-gap-analysis/11-collaboration.md (still open [codex] Add editorial grammar upgrade montage#1)

[explicit09]

Closing as a duplicate of #17. The bisect implementation is in draft PR #27.