fix(compression): recover rotated session lineage

[ruizanthony]

Summary

• prevent messages from being persisted to a parent session after compression has atomically rotated it
• recover and adopt the unique live compression child before the next turn continues
• fail closed when competing children make the continuation ambiguous
• keep the in-memory agent, flush cursors, cached system prompt, context engine, and durable transcript synchronized after adoption
• preserve parent workspace, git, user, and chat metadata when publishing the child session

Problem

A gateway or WebUI worker can remain bound to the parent session after another compression path has already rotated it. The existing duplicate-rotation guard correctly skips a second rotation, but the stale worker can then continue appending messages to the closed parent. Repeated turns grow the obsolete transcript until the provider context limit is reached.

Approach

• add an atomic persistence guard for sessions closed with end_reason=compression
• identify a canonical direct continuation child, excluding ended sessions and non-continuation branch/tool/delegation children
• return no candidate when more than one plausible continuation exists
• rebind stale agents at the start of a turn and reload the compacted transcript
• reset persistence cursors and notify context lifecycle hooks after rebinding
• reject stale pre-lease snapshots in concurrent compression paths

Validation

• compression/state/gateway regression matrix: 732 passed
• protected replay against current origin/main: 82 passed
• live protected replay smoke: 82 passed
• Python compilation checks passed
• git diff --check origin/main...HEAD passed

Operational evidence

The regression was reproduced from a parent session closed by compression that subsequently received hundreds of messages while its canonical child remained live. The fix is generic and contains no site-specific configuration or credentials.

[alt-glitch]

This was generated by AI during triage.

Related: #64373 hardens durable lease/atomic rotation. This PR additionally repairs stale-worker session adoption and gateway transcript rerouting after a rotation; reviewers should assess the complementary recovery path.

[egilewski]

too large to review safely

This PR changes 583 production lines before tests and docs. Please split it or add a focused justification if it should stay together.

Signed: GPT-5.6-terra-low in Codex

[teknium1]

Merged via #71001 with your authorship preserved — atomic child publication, closed-parent write guard, and live-child adoption all shipped. Follow-ups: publish_compression_child now inherits profile/thread/display/origin from the parent (the direct INSERT would have bypassed fork inheritance), and the pre-lease drift guard became rotation-only + length-based (content equality aborted in-place compaction and would wedge on legal in-memory mutation). Thanks for tackling the hardest subsystem in the sweep.