refactor: remove placeholders and share provider and HTTP plumbing

[Qiyuanqiii]

Summary

Remove five unreferenced mode placeholder packages, the forwarding agent.NewID helper, and an accidentally tracked PR draft. Retain the live Task compatibility model and update the Surface inventory references.

Execution profile, permission, and interaction controls now reuse the existing authorization helper. OpenAI transports share request-header construction; Responses and Anthropic share their existing SSE framing loop while keeping their distinct completion, tool-call, replay, and error handling.

Cleanup candidates were contributed by MiniMax-AI-Dev; the commit credits both MiniMax-AI-Dev and Codex.

Validation

• Passed go test ./internal/llm (local Go 1.26.6).
• Passed go test ./internal/httpapi -run '^Test(RunExecution(Profile|Permission|Interaction)Control|RunOperationControlsRejectDisabledUnauthorizedAndMalformedRequests)'.
• Passed CLI build and Surface registry/inventory validation against the main registry.
• Full Go 1.25 and platform checks use the existing PR CI.

Surface governance

• No Surface is added, promoted, downgraded, deprecated, or removed.
• Registry item(s): ctf-offensive-skeletons evidence references only; no tier or transition change.
• Target tier / transition: N/A — no Surface change.
• Entry criteria / decision: N/A — no Surface change.
• Owner: N/A — no Surface change.
• Shared Go Application contract: Existing application and provider contracts remain in place.
• Authority impact: None; the same endpoint gates and authorization order are retained.
• Supported platforms: Existing supported platforms, unchanged.
• Release / test evidence: Focused checks above and existing CI.
• Compatibility strategy: Keep historical task identifiers and provider protocol semantics.
• Deprecation window: N/A — no Surface change.
• Removal / rollback plan: Revert this source refactor; no database migration or data conversion.

Audit

• No credentials or local runtime data are included.
• Policy, workspace, sandbox, and persistence boundaries were reviewed.
• Architecture status and the generated Surface inventory are updated.