fix(cli): sanitize inherited Pi package directory

[rochecompaan]

Summary

• Sanitized executable Patchmill invocations by deleting inherited PI_PACKAGE_DIR before loading the bundled CLI graph.
• Added a fresh-process regression fixture that proves sanitization precedes CLI module evaluation, child processes inherit the sanitized environment, unrelated credentials/session metadata remain available, and exit-code propagation still works.

Validation

• node --test bin/patchmill.test.ts passed (2/2)
• npm test passed (1122 tests, 0 failures)
• npm run lint passed
• npm run build passed (emission check; TypeScript build has noCheck enabled)
• node dist/bin/patchmill.js --help passed
• PI_PACKAGE_DIR=/nix/store/outer-pi/libexec/pi "$package_path/bin/patchmill" --help passed after nix build .#patchmill --no-link --print-out-paths
• git diff --check passed; worktree clean

Reviews

• Final Codex full-worktree review: pass, no findings.
• Final thermo-nuclear full-worktree review: pass, no findings.
• Final validation-readiness review: pass, all required commands rerun.

Closes #117

Patchmill run cost

Stage	Model	Prompt tokens	Output tokens	Estimated cost (USD)
Development environment	gpt-5.5	321,202	1,700	$0.5857
Implementation	gpt-5.5	1,640,610	12,277	$1.6133
Implementation	gpt-5.6-sol	1,198,844	17,221	$1.9858
Implementation	gpt-5.6-terra	1,244,518	4,376	$0.5688
Implementation subtotal		4,083,972	33,874	$4.1679
Total		4,405,174	35,574	$4.7535

Prompt tokens include uncached input, cache reads, and cache writes. Cost is an estimate based on Pi's recorded model pricing and includes parent and subagent sessions.