feat: add prompt delivery to running sessions (send message / submit plan)

[jeremyeder]

Problem

The ACP MCP server can create sessions (acp_create_session) and read their state, but has no tool for sending prompts to a running session. The task field in CreateSessionRequest sets the CR spec but doesn't deliver the prompt to the runner — the runner starts and waits for AG-UI input.

Impact

• The acp-compile skill (submit a plan for execution) cannot deliver the plan to the session
• Users must manually send the first message through the UI
• No way to programmatically interact with a running session via MCP

Proposed Solution

Add an acp_send_message (or acp_submit_prompt) MCP tool that:

1. Takes session ID, project, and message (the prompt text)
2. POSTs to the public API's AG-UI proxy endpoint (see feat(public-api): add /v1/sessions/:id/runs endpoint for AG-UI prompt delivery platform#753)
3. Returns a summary of the response or streams events

This completes the autonomous workflow: create session → send prompt → monitor progress.

Additional Issues Found

During testing, the following MCP server issues were also observed:

• acp_login succeeds but subsequent calls still return 401 (token may not be persisted correctly across calls)
• acp_create_session project parameter is ignored — the server doesn't pass X-Ambient-Project header
• MCP server config points at K8s API server URL instead of public API URL for non-local clusters
• acp_list_sessions returns "Project required" even when project param is set

Context

Discovered while trying to use the acp-compile skill to submit a plan to a kind cluster session. Had to fall back to direct curl to the runner pod (bypassing the UI entirely) because no API path existed for prompt delivery.