fix: isolate Familiar Mix execution

[Oscar-Williams]

Problem

A rooted Familiar currently allowlists compile and format by default, so workspace Mix code can run before an operator opts in. Mix children also inherit the BEAM process environment, which can expose credentials to untrusted workspace code.

Fix

• Default the Familiar Mix allowlist to empty; run_tests: true explicitly opts into test.
• Start Mix children with a minimal environment while preserving caller-provided task variables.
• Add regressions and document the explicit trust boundary.

Validation

• mix test test/mix_gate_test.exs (9 passed)
• mix test test/familiar_test.exs:52 test/familiar_test.exs:64 test/familiar_test.exs:119 (3 passed)
• mix compile --warnings-as-errors
• mix format --check-formatted and mix credo --ignore refactor on a clean LF checkout

Closes #24