Server-owned agent completion: remove report_completion dependency

When an agent process exits normally, the server now automatically runs
acceptance gates (uncommitted changes check + cargo clippy + tests) and
advances the pipeline based on results. This replaces the previous model
where agents had to explicitly call report_completion as an MCP tool.

Changes:
- Add run_server_owned_completion() free function in agents.rs that runs
  gates on process exit, stores a CompletionReport, and advances pipeline
- Wire it into start_agent's spawned task (replaces simple status setting)
- Remove report_completion from MCP tools list and handler (mcp.rs)
- Update default_agent_prompt() to not reference report_completion
- Update all agent prompts in project.toml (supervisor, coders, qa,
  mergemaster) to reflect server-owned completion
- Add guard: skip gates if completion was already recorded (legacy path)
- Add 4 new tests for server-owned completion behavior
- Update tools_list test (26 tools, report_completion excluded)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

server/src/config.rs

@@ -61,8 +61,8 @@ fn default_agent_command() -> String {
 fn default_agent_prompt() -> String {
     "You are working in a git worktree on story {{story_id}}. \
      Read .story_kit/README.md to understand the dev process, then pick up the story. \
-     When all work is committed, call report_completion with story_id='{{story_id}}', \
-     agent_name='{{agent_name}}', and a brief summary as your final action."
+     Commit all your work when done — the server will automatically run acceptance \
+     gates (cargo clippy + tests) when your process exits."
         .to_string()
 }