[Phase 2] Implement guided promotion planning and dry-run flow

[phamhungptithcm]

Goal

Move from read-only analysis into guided promotion planning with safe defaults.

Scope

• gig promote
• promotion plan generation
• dry-run mode
• confirmation step before write actions
• early conflict checks
• clearer risk and warning output

Task Checklist

• define the promotion domain model and CLI contract for gig promote
• implement a promotion plan preview that shows candidate commits by ticket
• add dry-run behavior without destructive side effects
• implement confirmation gating before write actions
• add early conflict and branch-state checks
• document promotion flow, edge cases, and safety rules
• cover promote planning with unit and integration-style tests

Acceptance Criteria

• users can preview a promotion plan before any write action happens
• dry-run mode is safe and informative
• the command warns early when branch state or commit history looks risky
• destructive operations never occur silently

References

• docs/02-scope-and-phases.md
• docs/07-promotion-engine.md
• docs/13-roadmap.md