(sessions): follow /clear to the new transcript instead of listing it apart

[abate]

Problem

/clear makes the CLI open a new jsonl under a new session id. Nothing in it names the session it replaced (no forkedFrom; every record carries the new id), so detectSessionTransitions() never matched it: the open terminal stayed on the old row and the new conversation appeared as a separate, unattached sidebar entry once its first prompt made it indexable.

Fix

The CLI rewrites ~/.claude/sessions/<pid>.json with the new sessionId on /clear (verified on live state files). cliSessionState.clearOwner(newId, ptyPid) finds the live pid naming newId and walks its /proc parent chain up to the PTY:

• mine → re-key exactly like a fork (session-forked).
• other → another process's file; ignored.
• pending → state file not rewritten yet; rechecked for up to 60 s.
• unknown (no /proc: macOS/Windows) → matched only when this is the sole live Claude PTY in the folder.

readNewSessionSignals() recognises the file by its first non-bookkeeping user record (classifyUserText). The renderer adds a pending "New session" row for the new id, since a /clear transcript is not indexed until its first prompt. The cleared conversation stays in the list as a past, resumable session.

Docs: .ai/contexts/cli-session-state.md → "Owner of a /clear transcript".

Tests

• 8 new in test/session-transitions.test.js (mine / other / pending→mine / stale pending / unknown sole vs. two candidates / prompt-not-clear / post-fork clear / awaiting fork), 1 in test/cli-session-state.test.js (clearOwner verdicts).
• Lint 0 errors. Full suite: failures only in the CLI-state canary (stale state file from CLI 2.1.259), sandbox-wrapper and test-pr tests — environment-bound, untouched files.

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