fix(usage-limits): invalidate legacy Antigravity cache

[pitimon]

Summary

• stamp newly written Antigravity quota cache blocks with process_scope: current-user-v1
• fail closed for legacy, missing, or unknown provenance markers
• preserve the shared Claude cache block and existing expiry validation
• avoid storing username, UID, email-derived identity, or package-version coupling

Why

Before #138, Antigravity process discovery could read another user's process on a shared host and persist that account's email, plan, and quota. Scoping future process scans does not make already-written cache trustworthy. This one-time schema/provenance gate ensures only cache produced by current-user-scoped code can be served.

Verification

• strict RED/GREEN regression: new legacy-cache test fails on origin/main
• current marker fallback still works
• missing and unknown markers fail closed
• expired marked cache still follows reset validation
• live writes stamp the marker
• npm run ci:local passes: dashboard 320/320, root 1036/1036, all validators green
• independent review concern about unknown-marker and expiry specificity addressed in committed tests
• static scan: no secrets, debug artifacts, or dangerous execution added

Closes #139