ci: harden Avatar SDK PR evidence gate

[NWYLZW]

Summary

• Recompose Avatar SDK CI evidence v2 on protected main at 45dc71e0993494a66f29296ad1b79e5c1e591107.
• Harden fail-closed pull-request evidence validation while preserving required build-test-pack identity and the existing workflow contract.
• The first post-merge Avatar SDK CI run is intentionally full because the workflow and evidence helper changed.

Hosted verification expectations

• validate-sdk-ci must succeed on the new PR head.
• Required build-test-pack must execute on a fresh runner and succeed; it must not be skipped.
• The attempt-scoped evidence artifact must be unique, retained for 2 days, and bind the official workflow run, head SHA, integration parents/tree, and base/head repositories.
• The protected-main post-merge CI run must succeed before Deploy Avatar consumes its exact SHA.

Risk and rollout

• Ambiguous, stale, expired, malformed, oversized, or mismatched hosted evidence fails closed; required checks are not weakened.
• The old PR artifact based on 3a624059dc614c94678e475216b1cfaa61a877e9 is expected to be rejected against the new main. The prospective merge model was validated locally and is not a substitute for hosted evidence.
• After this PR, run a separate source-only hosted canary before relying on reused-pr-ci in production.

Validation

• Local evidence/workflow/deploy and new-main regression tests: 59 passed.
• Root and npm typechecks, all four SDK builds/typechecks, Node syntax, Ruby YAML duplicate-key/safe parse, and git diff --check passed.
• No npm publish, tag, ruleset, settings, environment, secret, permission, or package-byte changes.