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>

tests/integration/test_smolmachines_launch.py

@@ -38,11 +38,18 @@ from claude_bottle.manifest import Manifest
 from tests._docker import skip_unless_docker
 
 
+_AGENT_PROMPT = "You are demo. Be brief."
+
+
 def _minimal_manifest() -> Manifest:
     return Manifest.from_json_obj({
         "bottles": {"dev": {}},
         "agents": {
-            "demo": {"skills": [], "prompt": "", "bottle": "dev"},
+            "demo": {
+                "skills": [],
+                "prompt": _AGENT_PROMPT,
+                "bottle": "dev",
+            },
         },
     })
 
@@ -117,6 +124,15 @@ class TestSmolmachinesLaunch(unittest.TestCase):
             f"expected a connect-refusal message; got: {r.stdout!r}",
         )
 
+    def test_prompt_file_lands_in_guest(self):
+        # provision_prompt copies the host-side prompt.txt into the
+        # guest at /root/.claude-bottle-prompt.txt. The content
+        # must match what the manifest declared so claude-code's
+        # --append-system-prompt-file reads the right text.
+        r = self.bottle.exec("cat /root/.claude-bottle-prompt.txt")
+        self.assertEqual(0, r.returncode, msg=r.stderr)
+        self.assertEqual(_AGENT_PROMPT, r.stdout.rstrip("\n"))
+
     def test_egress_port_bypass_probe(self):
         # Agent dials <bundle-ip>:9099 (egress's port). TSI
         # permits the IP, but egress will bind 127.0.0.1:9099