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/prepare.py

@@ -13,6 +13,7 @@ from pathlib import Path
 from ...backend import BottleSpec
 from ...backend.docker.bottle_state import (
     BottleMetadata,
+    agent_state_dir,
     bottle_identity,
     write_metadata,
 )
@@ -85,8 +86,21 @@ def resolve_plan(
             f"http://{bundle_ip}:{_BUNDLE_SUPERVISE_PORT}"
         )
 
+    # Prompt file is always written (mode 0o600) so the in-VM
+    # path always exists. Content is the agent's `prompt`
+    # field (markdown body) — empty for agents with no prompt.
+    # claude-code reads it via --append-system-prompt-file only
+    # when non-empty, but the file must exist either way to
+    # match the docker backend's contract.
+    agent_dir = agent_state_dir(slug)
+    agent_dir.mkdir(parents=True, exist_ok=True)
+    prompt_file = agent_dir / "prompt.txt"
+    agent = manifest.agents[spec.agent_name]
+    prompt_file.write_text(agent.prompt or "")
+    prompt_file.chmod(0o600)
+
     machine_name = f"claude-bottle-{slug}"
-    # Chunk 2d placeholder until chunk 4's agent-image work lands.
+    # Chunk 2d placeholder until the agent-image work lands.
     # alpine pulls cleanly from docker.io via smolvm's crane
     # backend; the real claude-bottle image lives in the local
     # docker daemon and isn't reachable that way.
@@ -103,6 +117,7 @@ def resolve_plan(
         machine_name=machine_name,
         agent_from_path=agent_from_path,
         guest_env=guest_env,
+        prompt_file=prompt_file,
     )