fix: safely start deferred sprints

[numnx]

What changed

• Safely fast-forward an unchanged sprint branch from the current default branch when the sprint starts, while permanently preserving branches that already contain task work or have diverged.
• Add an idle sprint Update Branch action for the same guarded manual fast-forward.
• Automatically plan an unplanned sprint when Start is invoked, after branch preflight and before task dispatch.
• Add manage_sprints action followup to save an idle, unplanned draft without starting planning, and document the schedule-after-sprint flow for the Project Manager.
• Route scheduled starts through the normal orchestration path so deferred sprints receive the same branch and planning safeguards.
• Update canonical/public documentation and add backend, integration, contract, route, and dashboard coverage.

Why

Draft sprint branches can be created before their actual start time. Previously, they could remain pinned to an older default-branch commit, while follow-up sprint planning could also begin before the preceding sprint completed. This aligns branch freshness and planning with the actual sprint start boundary.

Validation

• pnpm run ci
• Dashboard/backend smoke: /health, /ready, and dashboard root responded successfully

Risk and rollback

Risk is concentrated in Git branch synchronization and sprint start sequencing. Updates use true fast-forward-only operations, reject divergence, and skip branches with execution history. The behavior is covered by real Git integration tests and orchestration tests.

Rollback: revert commit 79b3e5d.