Support client system prompt replacement and append

[salman1993]

Why

ACP clients cannot currently replace or append to Pi's system prompt when they create a session. Clients must use process wrappers or put standing instructions into user messages, which does not provide the same behavior.

What

Accept _meta.systemPrompt on session/new: a nonempty string replaces Pi's base prompt, and { "append": "..." } appends instructions. Advertise both modes and persistence through agentCapabilities._meta.piAcp.systemPrompt.

How

Validate the request before starting Pi. Pass literal prompt text through private files to Pi's existing CLI flags, then save the mode and text with the session record. Reapply that snapshot on automatic restoration and session/load, including after an adapter restart. Remove temporary files when the subprocess exits and saved prompts when sessions are deleted.

Omitting the extension preserves existing behavior. This change does not add skill or argument forwarding.

references

• Add support for appending or overriding the system prompt agentclientprotocol/claude-agent-acp#91
• fix: Forward full systemPrompt preset options from _meta agentclientprotocol/claude-agent-acp#591

Risk

The saved prompt lives in the adapter's session map, separately from Pi's transcript; restoration depends on retaining that map. Pi still adds its normal context, and explicit append supersedes automatic APPEND_SYSTEM.md discovery. Live checks ran on macOS; Windows process execution was not tested.

Testing

• PI_ACP_TEST_REAL_PI=1 node --import tsx --test test/component/pi-rpc-system-prompt.test.ts passed against Pi 0.84.4. Its RPC HTML export contained the custom prompt exactly once, and only append retained Pi's native coding preamble. No model calls were made.
• Exercised the built adapter over JSON-RPC stdio with isolated HOME and Pi settings. Created replacement session A and append session B, seeded synthetic transcripts, and inspected exports through session/prompt. Automatic restoration of A, explicit loading of B, and restoration after a full adapter restart all preserved the correct prompt without cross-session leakage. No model calls were made.

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