docs: make the first-lease quickstart executable

[FetchDoge]

Summary

Repairs the first-use guide without changing runtime behavior:

• Adds Python >=3.11, virtual-environment and local package-install prerequisites.
• Imports Path and exception classes, assigns the reported fault, catches expected refusals, and demonstrates clear/resume.
• Removes an unmeasured five-minute claim and states the admission-only boundary: not an OS sandbox and not interruption of already-running work.

Verification

• Installed the package in a scratch virtual environment using supported Python 3.14.7; ordinary python -m pip install . succeeded. The literal python3.11 binary was not available for this check.
• Executed every guide Python fence in order: exit 0.
• Executed the original guide as a negative control: exit 1, undefined Path.
• Existing Interlock suite: 50 passed.
• Whitespace/local markdown-link checks and patch application check passed.

Local execution is not an independent stranger onboarding test. No new runtime capability, adoption, or production-safety claim.

Scope and attribution

Documentation-only repair. Marcus Richards directed this adoption/onboarding slice; AI-assisted implementation used Codex after a Cursor consult timed out, with coordinator execution/diff verification. No source behavior changes. Separate approval is required for merge.