Settle forked-skill subagents from their own transcript

[calvintvu]

Context

A subagent kept the running badge after its session closed. The subagent came from a forked skill, for example /code-review. No tool call spawns a forked skill, so its meta.json carries no toolUseId. subagentStatus matched that empty key against the completed tool results of the parent, and the match never succeeded.

Changes

• When its own transcript is complete, a forked-skill subagent now reads returned. Before this change it read running forever.
• The fallback needs an empty spawn key. A normal Task subagent keeps the tool-result path, which stays authoritative. That path settles a child only after the parent writes the tool result, so a child mid-handoff never flickers to returned.
• InTurn is the signal, and it already clears on a terminal stop_reason (claude/parse.go:275). The fix reads a value the parser produces today and adds no new parsing.
• Note: the digest gap remains. linkSpawnDigest still logs has no spawn row in the parent transcript for a forked skill, so the subagent shows in the rail with no turn anchor.

Test

• go vet ./... — passed
• go test ./... — passed
• Manual: List(0) over the local transcripts. Subagent agent-a5101e8ad9c72bdc6 in session cebc97ec reads returned. It read running before the fix.

Screenshots

with session closed
before

after