fix(server): report Prime plan observation

[rynfar]

Problem

Prime already sends bounded plan progress through the managed daemon integration and ACP PlanUpdated events, but its provider capability snapshot said planning was entirely unavailable. That made the machine-facing feature inventory disagree with shipped behavior.

Fix

• Advertise Prime planning as read-only observe in daemon and ACP modes.
• Keep formal Plan interaction mode unavailable, hidden, and rejected by both adapters.
• Clarify the internal parity ledger and user limitations without adding hidden prompts or write controls.

Validation

• vp test run apps/server/src/provider/prime/PrimeAgentFeatureCapabilities.test.ts apps/server/src/provider/Layers/PrimeAgentProvider.test.ts — 24 passed
• vp run -F t3 --fail-if-no-match typecheck
• targeted lint and format checks
• git diff --check

Browser verification was not needed because the existing Plan/Build control remains hidden for Prime.

Model: openai-codex/gpt-5.6-sol
Harness: Prime Agent RLM

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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	13.6 KiB	13.6 KiB	+14 B (+0.1%)	15.1 KiB	✅
Codex	Thread snapshot wire	6.9 KiB	6.9 KiB	+2 B (+0.0%)	7.3 KiB	✅
Codex	Live turn WebSocket wire	6.6 KiB	6.6 KiB	+12 B (+0.2%)	7.8 KiB	✅
Codex	Live turn WebSocket decoded	57.2 KiB	57.2 KiB	0 B (0.0%)	66.4 KiB	✅
Codex	Live turn messages	10	10	0 (0.0%)	21	✅
Claude	Total thread wire	13.6 KiB	13.7 KiB	+170 B (+1.2%)	15.1 KiB	✅
Claude	Thread snapshot wire	6.9 KiB	6.9 KiB	−8 B (−0.1%)	7.3 KiB	✅
Claude	Live turn WebSocket wire	6.6 KiB	6.8 KiB	+178 B (+2.6%)	7.8 KiB	✅
Claude	Live turn WebSocket decoded	58.1 KiB	59.5 KiB	+1.5 KiB (+2.5%)	66.4 KiB	✅
Claude	Live turn messages	10	11	+1 (+10.0%)	21	✅

Baseline: 4ce0070 · PR result: b51abb0 · 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: 109.5 KiB
• Claude decoded thread snapshot: 110.2 KiB

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