fix(clients): ignore stale compaction after session stops

[rynfar]

Problem

A completed Prime Agent thread could become permanently unsendable after its session stopped. If the provider missed a compaction terminal event, the web composer kept treating the last persisted compacting activity as live even though no active session remained, so Send was disabled without a surfaced error.

Fix

Require an active, compaction-capable session scope before compaction state can block composer submission. The shared guard is used by web and mobile, and the regression test covers a stopped session with stale compacting activity.

Verification

• vp test run packages/client-runtime/src/state/contextCompaction.test.ts (7/7)
• vp run -F @t3tools/client-runtime typecheck
• vp run -F @t3tools/web typecheck
• vp run -F @t3tools/mobile typecheck
• targeted vp lint on all four changed files
• targeted vp fmt --check on all four changed files
• git diff --check

Integrated browser evidence is pending explicit permission for browser use.

Implemented with GPT-5.6 Sol using the Prime Agent harness.

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[github-actions]

Thread transfer impact

✅ Thread transfer remains within every enforced ceiling.

Provider	Metric	Main baseline	This PR	Impact	PR ceiling
Codex	Total thread wire	13.3 KiB	13.3 KiB	−4 B (−0.0%)	15.1 KiB	✅
Codex	Thread snapshot wire	6.9 KiB	6.9 KiB	+6 B (+0.1%)	7.3 KiB	✅
Codex	Live turn WebSocket wire	6.4 KiB	6.4 KiB	−10 B (−0.2%)	7.8 KiB	✅
Codex	Live turn WebSocket decoded	55.5 KiB	55.5 KiB	0 B (0.0%)	66.4 KiB	✅
Codex	Live turn messages	9	9	0 (0.0%)	21	✅
Claude	Total thread wire	13.3 KiB	13.3 KiB	−28 B (−0.2%)	15.1 KiB	✅
Claude	Thread snapshot wire	6.9 KiB	6.9 KiB	+1 B (+0.0%)	7.3 KiB	✅
Claude	Live turn WebSocket wire	6.4 KiB	6.4 KiB	−29 B (−0.4%)	7.8 KiB	✅
Claude	Live turn WebSocket decoded	56.4 KiB	56.3 KiB	−44 B (−0.1%)	66.4 KiB	✅
Claude	Live turn messages	10	9	−1 (−10.0%)	21	✅

Baseline: 2b7a9b8 · PR result: 1b83337 · Source CI: success

Scenario and decoded snapshot size

10 historical turns, 5 command tools per turn, 878.9 KiB retained MCP result per historical turn, and a 1.05 MiB retained result in the measured turn.

• Codex decoded thread snapshot: 109.4 KiB
• Claude decoded thread snapshot: 110.1 KiB

Updated in place by a trusted workflow. PR artifacts are strictly validated and never executed.