feat(acp): implement session/load so BB threads resume with history

[masked8knights]

Supersedes #1600 (reopened from the rebased branch, based on current main).

Problem

When BB resumes a chat thread through its ACP provider, it calls session/load with the thread's stored provider thread id. It only restores in-agent history when the agent advertises loadSession and answers session/load.

prime-agent's ACP mode advertised loadSession: false and had no session/load handler, so every restarted BB conversation fell back to session/new with blank in-agent history, and BB showed:

could not restore the previous session; continuing in a fresh session without in-agent history.

That's the "Prime Agent in BB keeps resetting / losing context" symptom.

Fix

• Advertise loadSession: true in the ACP initialize response.
• session/new persists the ACP session id → prime-agent JSONL session file mapping under the agent dir (<agentDir>/acp-sessions/<id>.json).
• session/load resolves the requested id to the saved transcript, switches the daemon connection back onto that session file, then admits the ACP session as usual — so subsequent prompts continue the restored conversation instead of starting fresh.
• A missing mapping or missing file throws, letting the client fall back to a fresh session with its own warning rather than pretending history was restored.
• Based on and compatible with the ACP MCP support (feat(coding-agent): support ACP MCP programs #1378); session admission is shared between session/new and session/load.

Verification

• New end-to-end test resumes a mapped session file across two ACP connections.
• 74/74 ACP-related tests pass (acp-mode, acp-features, acp-stop-reason, acp-events).

Note

Implement ACP session/load to resume BB threads with persisted session history

• Adds a session/load request handler that resolves an ACP session id to a saved JSONL session file and switches the underlying connection to that file, restoring prior conversation history.
• On session/new, the agent now writes an on-disk mapping (ACP session id → session file) under acp-sessions/ in the agent directory, using atomic rename with 0700 permissions.
• initialize now advertises agentCapabilities.loadSession = true.
• Extracts session admission logic (event subscription, initial snapshot reconciliation, gating) into a shared admitAcpSession helper used by both session/new and session/load.
• Risk: session/load fails if no mapping file exists on disk for the given sessionId; mapping persistence/lookup errors are swallowed, so a missing or corrupt mapping results in a load failure rather than a crash. Reviewers should check resolveAcpSessionFile in acp-mode.ts and the PRIME_AGENT_CODING_AGENT_DIR fallback in acpSessionLinkDir.

^{Macroscope summarized 9a338b4.}

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