Close CMUX fleet Git probe environment

[teamleaderleo]

Follow-up to the merged CMUX fleet acceptance hardening (#1088 / #1091).

The local acceptance children already use isolated Python and an explicit allowlist, but the pre-run source identity probe still launched /usr/bin/git rev-parse with almost the entire ambient operator environment.

This repair:

• starts the Git probe from an explicit four-variable environment;
• forces C locale;
• disables global/system Git configuration;
• excludes PATH/HOME/Python/SSH/secret/loader/Git override variables;
• adds a hostile-environment regression proving the exact child environment.

This changes no CMUX workload semantics, fleet role policy, routing preference, or dispatch authority.

Refs #1056
Refs #546

[teamleaderleo]

Exact-head repair handoff: 396a43177639791a8c0416db5a850e5d1c446319.

The remaining CMUX local-acceptance child-process leak is closed narrowly:

• /usr/bin/git rev-parse now starts from an explicit four-variable environment;
• locale is fixed to C;
• global/system Git configuration is disabled;
• ambient PATH/HOME/Python/SSH/loader/secret/Git override variables are absent;
• regression coverage injects those hostile ambient values and verifies the exact child environment.

Verify 35675608842: success.
Cultist advisory 35675608849: success.

This changes source-identity observation only; CMUX profile semantics, fleet candidacy, routing preference, and dispatch authority are unchanged.