Toolchain readiness host smoke tests

[BramVR]

Parent: #70

What to build

Add host toolchain readiness smoke tests for codemesh doctor and agent preparation. These should verify real binaries when available while also using controlled PATH fixtures to test missing-tool behavior deterministically.

Acceptance criteria

• Tests cover at least one Go project fixture and one script/package-manager-style fixture once Toolchain Readiness exists.
• Present-tool cases record detected command names and versions in the report without installing or modifying tools.
• Missing-tool cases use a controlled PATH to prove readiness diagnostics and strict-mode blocking.
• doctor and agent prepare agree on toolchain readiness results through the shared readiness model.
• Live host detections become SKIP when optional tools are absent, unless strict mode is requested.
• The report separates host facts from project facts so agents can tell whether a failure is machine setup or CodeMesh behavior.
• Docs state clearly that CodeMesh observes toolchains and does not build environments in the MVP.

Blocked by

• Live e2e harness guardrails and report mode #88
• Toolchain Readiness in doctor and contract #86

Cost guardrail

• Must stay free: observe host tools already present; do not install paid tools, provision environments, or require paid runners. Missing optional tools SKIP unless strict mode is requested.