feat: auto-merge a run's session into one file at exit

[lroolle]

A resumed session fragments across one trace-*.jsonl per run; the viewer stitches them at read time, but the disk never converges unless someone remembers cctrace merge --yes. Now every capture run consolidates its OWN sessions on the way out.

What it does

• At exit (after flush, before the receipt names any file), the run collects the session ids it saw on the wire and runs a session-scoped merge: union into session-.jsonl, prune fully-subsumed sources.
• Fires only when there is something to consolidate: >1 source file for the sid, or an existing session-* output. A fresh single-file session's trace stays byte-identical.
• When the run's own trace is absorbed, the "Reopen anytime" line and the registry tombstone point at the session file instead of a path that no longer exists.
• --no-auto-merge opts out of just this; --fresh opts out of the whole continuity layer. Legacy node mode untouched. Works for every client (claude/codex/grok/kimi) via extractSessionId.

Correctness

• planMerge(logDir, { sessionIds, fragmentedOnly }): scoped prune-ability is stricter than the whole-dir sweep -- a source is prunable only when every pair lands in a session THIS plan writes. A file also holding an unmerged session's pairs (e.g. a concurrent run) is never pruned. Regression-tested.
• Whole-dir cctrace merge behavior is unchanged (with all sessions written the new predicate reduces to the old one); existing merge tests pass untouched.
• Keeps applyMerge's discipline: atomic writes, union-never-shrink, re-stat before each unlink (a live capture appending mid-merge is skipped, not truncated).
• Fail-soft: wrapped so a merge error logs one dim line and never costs the exit receipt or exit code.

Verification

• bun test: 578 pass, 0 fail (7 new scoped-merge tests + flag parse test).
• End-to-end with a fake client through the base-url proxy: run 1 untouched -> run 2 merged 2 sources and pruned both -> run 3 grew the existing session file. Both opt-outs suppress it; manual merge --prune --yes output unchanged.

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