feat(cli): add headless launch mode for orchestrators

`--headless` is a non-interactive launch path for `cli.py start`:
agent, bottles, label, and color come from flags + manifest defaults
with no TUI selectors and no y/N preflight (auto-confirmed via a new
`assume_yes` param threaded into the shared `_launch_bottle` core).

- `--bottle` (repeatable) defaults to the agent's own `bottle:`;
  `--label` defaults to the agent name and auto-uniquifies on slug
  collision; `--color` defaults to none.
- `--prompt TEXT` is required in headless mode and is delivered to the
  agent via a new `headless_prompt(prompt)` method on `AgentProvider`,
  implemented for claude (`-p`), codex (positional), and pi (`-p`).
- The agent still execs on inherited stdio/PTY, so whatever allocates
  the PTY drives the live session; only the launch chrome is headless.
- `--headless --dry-run` previews the resolved plan without launching.

Adds unit coverage in tests/unit/test_cli_start_headless.py and
headless_prompt tests for each provider. Also stubs headless_prompt on
the in-test AgentProvider subclasses so the unit suite collects cleanly.

Closes #315.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01WL77TgFxKbs3cidGMG9dz7

bot_bottle/contrib/claude/agent_provider.py

@@ -313,6 +313,9 @@ class ClaudeAgentProvider(AgentProvider):
                 f"claude mcp add --scope user --transport http supervise {supervise_url}"
             )
 
+    def headless_prompt(self, prompt: str) -> list[str]:
+        return ["-p", prompt]
+
 
 def _exec(bottle: "Bottle", script: str, error: str) -> None:
     result = bottle.exec(script, user="root")