ACP: use session/load|resume for warm agent sessions instead of the replay-preamble hack

[jonathanKingston]

Context

The first real npm run probe:acp run (Tier-1 capability probe, PR #829) shows the two main ACP adapters already advertise session resume:

Capability	Claude (claude-agent-acp 0.53.0)	Codex (codex-acp 1.1.0)	Cursor
Session load	✓	✓	✓
session/resume	✓	✓	·
session/list	✓	✓	✓
session/fork (unstable)	✓	·	·

But Copse's ACP session pool (acp-session-pool.ts / acp-client.ts) does not use these. Per docs/acp-agents.md, a session idle for 10 minutes is reaped and, on respawn, "the prior conversation is replayed once as a compact preamble" — a lossy, token-costly reconstruction that also feeds the cost-multiplier concern in #584.

Proposal

For adapters that advertise loadSession / session/resume, resume the real agent session (agent-owned history) instead of replaying a synthesized preamble:

• On respawn after idle reap or a config change, call session/load (or session/resume where advertised) with the stored sessionId rather than replaying the transcript.
• Fall back to the current replay path for agents that advertise neither (e.g. Cursor advertises only legacy loadSession, no resume).

Evidence / refs

• Findings: docs/acp-support-findings.md (added in PR Add ACP capability probe (Tier-1 agent evaluation) #829)
• Probe: docs/acp-capability-probe.md — reproduce with npm run probe:acp
• Related: Secondary LLM cost multipliers (ACP transcript replay, no cache key, unconditional subagents) #584 (transcript-replay cost)

Surfaced by the Tier-1 ACP capability probe.