feat(smolmachines): provision_prompt + provision_skills (PRD 0023 chunk 4a)

First slice of chunk 4: implement the two provisioning methods
that don't depend on agent-image tooling beyond `cp` and
`mkdir`. provision_ca / provision_git / provision_supervise
land once the agent-image gap is solved (chunk 4b+) — they need
update-ca-certificates, git, and the claude binary respectively,
none of which the chunk-2d alpine placeholder provides.

What this PR ships:

- `claude_bottle/backend/smolmachines/provision/` subpackage
  with `prompt.py` + `skills.py`. Each routes through
  `smolvm.machine_cp` / `machine_exec`. provision_prompt mirrors
  the docker contract (file always copied; return value drives
  --append-system-prompt-file iff the agent has a non-empty
  prompt). provision_skills mkdir + cp per skill, matching
  the docker backend's loop.
- prepare.py now writes the prompt file under
  agent_state_dir(slug) with the agent's `prompt` body, mode
  0o600. The in-guest path is `/root/.claude-bottle-prompt.txt`
  (alpine has no `node` user; will become `/home/node/...` once
  the real claude-bottle image lands).
- launch.py calls `provision(plan, machine_name)` after
  machine_start. The returned prompt path threads to
  SmolmachinesBottle so exec_claude can add
  --append-system-prompt-file when the agent has a prompt.
- backend.py: provision_prompt / provision_skills now real;
  provision_git is a deliberate stub (waiting on the git-gate
  inner Plan + git in the agent image). provision_supervise
  stays the chunk-2d stub.

Tests:
- 7 new unit cases (test_smolmachines_provision.py): argv
  shape (mocked smolvm.machine_cp / .machine_exec),
  prompt return-value contract, no-op-with-no-skills,
  CLAUDE_BOTTLE_GUEST_SKILLS_DIR override, fail-on-missing-skill.
- 1 new integration case in test_smolmachines_launch.py:
  end-to-end verification that the prompt file lands in the
  alpine guest at /root/.claude-bottle-prompt.txt with the
  expected content (via `bottle.exec("cat ...")`). The smoke +
  the two TSI probes stay green.

552 unit + 4 integration (Darwin+smolvm+docker gated) passing.

What's left in chunk 4:
- 4b: thread the inner Plans (PipelockProxyPlan / EgressPlan /
  GitGatePlan / SupervisePlan) through prepare + launch so the
  bundle daemons actually run (currently daemons_csv="").
- 4c: the agent-image-conversion gap — get claude-code + git +
  curl + ca-certificates into the guest image (build a
  .smolmachine via `pack create --from-vm` after manual setup,
  or push the docker image to a registry smolvm can pull).
- 4d: provision_ca + provision_git + provision_supervise once
  4b + 4c land.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

claude_bottle/backend/smolmachines/bottle.py

@@ -21,8 +21,12 @@ class SmolmachinesBottle(Bottle):
     on the launch ExitStack — this class only routes runtime
     operations to the right `smolvm machine ...` subcommand."""
 
-    def __init__(self, machine_name: str) -> None:
+    def __init__(self, machine_name: str, *, prompt_path: str | None = None) -> None:
         self.name = machine_name
+        # In-VM path to the agent's prompt file. None when the
+        # agent declared no prompt (file still exists; we just
+        # don't pass --append-system-prompt-file).
+        self._prompt_path = prompt_path
 
     def exec_claude(self, argv: list[str], *, tty: bool = True) -> int:
         """Run `claude` interactively inside the VM. Inherits the
@@ -37,7 +41,10 @@ class SmolmachinesBottle(Bottle):
         flags = ["smolvm", "machine", "exec", "--name", self.name]
         if tty:
             flags += ["-i", "-t"]
-        flags += ["--", "claude", *argv]
+        claude_argv = ["claude"]
+        if self._prompt_path:
+            claude_argv += ["--append-system-prompt-file", self._prompt_path]
+        flags += ["--", *claude_argv, *argv]
         result = subprocess.run(flags, check=False)
         return result.returncode