fix: preserve fork cost boundaries without copying parent events

[Brian Krabach (bkrabach)]

What changed

• Stops legacy fork copies of parent root events, CI capture, and lifecycle metadata; child sessions retain only their own event activity. No session data is deleted.
• Persists a versioned immutable fork_cost_boundary in child native metadata, so cost reporting accounts for parent-through-fork history plus child-owned cost without reconstructing ancestors.
• Maps Foundation-native turns conservatively across reminder/list envelopes and repeated owner suffixes, and explicitly refuses an ambiguous intra-prompt cutoff rather than claiming a false boundary.
• Preserves normal root-resume compatibility and keeps --no-events as a compatibility option.

Why

Forked sessions need accurate inherited-cost accounting without copying parent-owned artifacts or assigning ownership incorrectly.

Verification

• Fable (claude-fable-5-1) reviewed exact head 4e1688d2d53daeb5e03ae710d2eb9b9e22d2ee27 and approved it after the final test condition was met.
• DTU with frozen dependencies: focused suite passed (71 tests); full suite passed (2,433 passed, 4 skipped, 13 deselected, 1 expected failure). The full suite was independently rerun.
• Deterministic rendered interactive fork/resume coverage verifies preserved metadata, reminder/repeated-turn mapping, nested ownership, and explicit unavailable boundaries for ambiguous cuts.
• Compaction coverage verifies preserved costs and the missing-CI warning path.

Limitations / caveats

• Historical forks without a stored boundary report incomplete history; the implementation does not guess retroactive ancestry.
• An explicit historical cut inside a reminder-split prompt can be unavailable; unaffected full-copy forks retain their boundary.
• Large-capture full scans and conservative cost caps remain in place.
• No external paid-provider validation was run; optional-behavior proof is limited by the partial-bundle fixture.

Breaking changes

None.