Collapse background re-homed Claude sessions into one card

[calvintvu]

Context

Claude Code can move a terminal session onto its background surface a short time after the session starts. It writes a second transcript that copies the conversation of the first, under a new session id. coSlash reads every root transcript as its own session, so one conversation renders as two identical cards and inflates the active-session count.

Changes

• A background transcript that holds every conversation row of another root now supersedes that root. Only the newer card remains, and it keeps the session id that Resume needs.
• The tokens of the superseded transcript fold into the survivor, so the cost of the pair does not change.
• Subagent transcripts re-point to the surviving session, so they do not lose their parent.
• A deliberate claude --resume branch keeps its own card. A re-home of such a branch supersedes only the branch, not the root it resumed from.

Test

• go test ./... — passed
• go vet ./... — passed
• Manual: ran the pass over a local transcript store of 79 root transcripts. It collapsed two re-home pairs and left two claude --resume pairs separate.

Screenshots

• Session list for a project with a re-homed session — BEFORE.

• Session list for a project with a re-homed session — AFTER.

[luhe19001]

Reviewed the re-home collapse approach — the uuid-containment + bg-only successor rules, token fold after applyForkedUsage, and fixture coverage look solid for the board/list path.

One follow-up on the preview family path (inline); otherwise LGTM for the stated goal.