Harden capability routing and release checks

[tetracoralla]

What changed

• added capability-provider drift checking to the ordinary release gate
• tightened the one-call Agent route for supported standard expressions
• kept Cron platform detection unresolved until the caller chooses a dialect
• clarified invalid SemVer as an evaluation error rather than a negative match
• added a core-boundary regression for illegal fields behind the flat host schema

Why

Fresh Codex tasks could enumerate nonexistent resources, send operation-incompatible fields, or turn structured errors into model conclusions. The provider manifest could also drift while the main check stayed green.

Impact

The public surface remains one read-only sei_run tool. The host-compatible schema stays compact while the deterministic core continues to reject invalid operation combinations.

Validation

• npm run check: 11 test files / 139 tests plus schema, safety, capability, Agent bundle, and fresh-package checks
• capability pilot suite passed
• installed bundle is byte-identical to source
• fresh Codex SemVer task used exactly one sei_run call