Add Drive file access to agent skills

[mbakgun]

Summary

Adds opt-in Drive file access for Agent skills, removes the Skill Builder's special 2 MB attachment-context gate, and lets Skill Builder fine-tune comments include uploaded files.

Changes

• Added per-skill driveFilesEnabled workflow JSON support and an Agent skill-card toggle.
• Added a generated heym_drive.py helper for enabled skill runs with file lookup/read helpers by Drive file id or filename.
• Scoped skill Drive reads to files accessible to the workflow actor, including owned and team-shared Drive files.
• Extended Skill Builder requests with per-message attachments, plus frontend drag/drop uploads that persist as skill assets.
• Added PDF/text/DOCX attachment context handling without a custom 2 MB Skill Builder limit.
• Updated workflow DSL guidance and docs for Agent skills and Drive.

Validation

• SECRET_KEY=test-secret-key-for-tests-only-32-bytes ./check.sh
• Focused backend tests for Skill Builder attachments, skill Drive helper runtime behavior, and agent skill tool metadata.

Notes

Frontend lint still reports two pre-existing vue/no-v-html warnings in CodexFollowupView.vue; the full check passes.