F-A Slice 2: Retire MutatingForker and the mutating fork capability #559

Parent

#538 (agent provider + handoff architecture overhaul). This is the F-A follow-up: retiring the Cursor Path A mutating fork.

What to build

Remove the mutating fork path entirely now that Cursor forks cleanly. This is mechanical cleanup once Slice 1 (#558) has proven the clean side-channel leaves the parent untouched.

Delete the MutatingForker adapter and the Cursor provider's runHiddenTurn method (and its hidden-turn / disregard-turn machinery). Retire the "mutating" value from the SessionForkBehavior capability so it becomes "clean" | "unsupported", and set Cursor's declared capability to "clean". The handoff ladder reduces from B/A/D to B/D.

Update the domain glossary in CONTEXT.md to match: remove the Path A retirement-pending note on the B/A/D ladder, remove the "Hidden turn" and "Disregard turn" and "Path A" definitions, and reframe the ladder as B/D. Remove or update any handoff.json provenance and fallback-banner copy that referenced the "A" ladder step, including tests asserting the mutating path.

The end-to-end behaviour is unchanged from the user's point of view: Cursor forks still produce a handoff. The only difference is there is no longer a mutating code path in the tree.

Acceptance criteria

• MutatingForker is deleted; no provider wires it.
• Cursor's runHiddenTurn and its hidden-turn / disregard-turn logic are deleted.
• SessionForkBehavior no longer includes "mutating"; Cursor declares "clean".
• The handoff pipeline and any provenance / banner copy no longer reference the "A" ladder step.
• CONTEXT.md reflects the B/D ladder: Path A retirement note removed, "Hidden turn" and "Disregard turn" terms removed.
• Existing tests that asserted the mutating path are removed or rewritten; forking a Cursor thread still produces a valid handoff.
• bun run verify passes.

Blocked by

#558 (must complete and its clean-parent verification must pass).