feat(cli): pearlite bootstrap subcommand wires Engine::bootstrap

[UnbreakableMJ]

Summary

Surfaces ADR-0012's bootstrap entry point to operators. Verifies the operator-supplied Determinate installer script against the host's declared SHA-256 pin, runs the installer if nix is missing, and writes /etc/nix/nix.conf with the experimental-features minimum (ADR-0013).

• New Command::Bootstrap { host_file, installer_script, nix_conf } in args.rs. installer_script is required (operator pre-downloads — URL-fetch is a follow-up). nix_conf defaults to /etc/nix/nix.conf.

• dispatch_bootstrap calls Engine::bootstrap with the per-host declared SHA and renders BootstrapOutcome in the envelope data.

• bootstrap_error_payload maps every BootstrapError variant to a typed error code with a runnable hint:

  • NIX_NOT_DECLARED — host has no [nix.installer] block
  • NIX_INSTALLER_SHA256_MISMATCH — declared vs. observed SHA (ADR-004)
  • NIX_INSTALLER_FAILED — script exec, missing shell, or non-zero exit
  • BOOTSTRAP_NIX_CONF_WRITE_FAILED / BOOTSTRAP_NIX_CONF_READ_FAILED
  • BOOTSTRAP_NICKEL_FAILED

  All are class=preflight, exit=2 — bootstrap halts before mutating state, so even installer failures are recoverable from the operator's perspective.

• RunContext gains nix_installer: Box<dyn NixInstaller>. main.rs wires LiveNixInstaller; the six dispatch test sites wire MockNixInstaller. Two new dispatch tests cover the happy path and the NIX_NOT_DECLARED error.

The next chunk lands the apply-side preflight check (NIX_NOT_INSTALLED when home_manager.enabled = true and nix --version fails — per ADR-0012 decision 3).

Test plan

• cargo test --workspace --all-features — 314 passing (+2 new)
• cargo clippy --workspace --all-targets --all-features -- -D warnings — clean
• cargo fmt --all -- --check — clean
• scripts/ci/check-spdx.sh — clean
• pearlite-audit check . — 1 check, 0 violations
• CI green on T1 / T2 / T3

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