docs: reconcile v0.26.0 public API contracts

[shiyubi]

Summary

• Updated OpenAPI metadata to v0.26.0 and regenerated related documentation assets.
• Corrected task ID types, nullable child-task IDs, profile-matching fields, and response schemas.
• Documented HTTP 200 business-error envelopes, validation responses, rate limits, and identifier caveats.
• Updated guides, endpoint inventory, response-source documentation, and changelog.