fix(mojo): isolate PM2 metadata from child environments

[swtxbling]

Summary

• scrub PM2-managed process metadata from the ambient environment before spawning Mojo children
• preserve PATH, HOME, proxies, certificates, credentials, and Botmux session routing; explicit bot env and mojo.env keep their existing precedence
• use the same effective environment for worker wrapper resolution and the real MojoBackend spawn path
• cover host, cloud, sandbox, dynamic instance_var safety, explicit overrides, and non-Mojo compatibility
• update the bug ledger with real-device verification and final review evidence

Validation

• Mojo 1.0.12 direct host Bash: 2/2 completed, one tool call each, return code 0
• real MojoBackend host smoke: completed with Bash result and final answer
• full Mojo Vitest suite: 33 files passed, 1 skipped; 667 tests passed, 32 skipped
• focused independent review: 140 passed, 1 skipped across the directly relevant suites
• bun 1.4.2 build: passed, including TypeScript checks, dashboard bundle, dist audit, and embedded-asset audit
• git diff --check: passed
• independent final review: PASS, blocker 0, major 0, minor 0 after ledger closeout

Compatibility and evidence boundary

• non-Mojo environment merging is unchanged
• ambient proxy variables remain available and can still be overridden or cleared explicitly
• PM2 Common.js drift coverage runs when PM2 is installed; static fixtures always cover the scrub behavior now that PM2 is no longer a runtime dependency
• on macOS, the isolated smoke workspace has no remaining process using it, but one synthetic-session containment handle remains fail-closed because Darwin cannot provide Linux /proc-level process-tree proof

Tracking

• BUG-20260828-001