Align Cloud Agent with Mission Control PR-decouple behavior

The VS Code Cloud Agent experience was out of sync with Mission Control's
CCA PR-decoupling flow. This aligns VS Code with MC behavior:

- Drop the delegate / commit / push modal. Codex and Claude don't show
  one; Mission Control doesn't either. Permissive GitHub auth now uses
  the standard non-modal sign-in prompt via createIfNone.
- Send the raw user prompt as problem_statement (no chat-history
  summary injection). This fixes the 'agent invents its own prompt'
  behavior Luke reported.
- Send a minimal pull_request block (title/base_ref/[head_ref]) to
  match what sweagentd + Mission Control send. Drop body_placeholder
  and the 'Created from VS Code' body_suffix.
- Drop the pre-delegation git preflight (uncommitted-changes detection,
  remote base-branch check, branch push). delegate() already falls
  back to the repo's default_branch when base_ref is not supplied.
- Delete the now-unreachable CopilotCloudGitOperationsManager and
  the unused body_suffix / formatBodyPlaceholder helpers.

Server-side verified: sweagentd v1 API (docs/adr/0001-create-job-api.md)
makes pull_request optional and returns job_id + session_id before the
PR exists; Mission Control's sweagentd client sends the minimal
pull_request shape used here. Full no-PR (non-PR) workflow is on the
sweagentd roadmap; this change is compatible with it.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: Oregand