Add ULID format validation to all v3 plan commands to prevent users from
accidentally mixing legacy ('agents tell') and v3 ('agents plan use')
workflows. The two systems use separate storage backends and cannot be
mixed; this change detects the mismatch early and provides actionable
error messages.
Changes:
- Add _validate_plan_ulid() with proper Crockford Base32 regex
(^[0-9A-HJKMNP-TV-Z]{26}$, re.IGNORECASE) that correctly rejects
invalid characters (I, L, O, U), hyphens, and wrong-length strings
- Add _PLAN_ULID_RE compiled regex and _ULID_VALIDATION_ERROR_MSG constant
with actionable guidance explaining the legacy/v3 incompatibility
- Apply ULID validation to all v3 commands: execute_plan,
_lifecycle_apply_with_id, lifecycle_apply_plan, plan_status,
plan_errors, cancel_plan (only on user-provided IDs, not auto-discovered)
- Update _LEGACY_DEPRECATION_MSG and tell/build command warnings to
explicitly state that the two workflows are INCOMPATIBLE and cannot be mixed
- Add comprehensive BDD tests in features/plan_ulid_validation.feature
with step definitions using Typer CLI runner (not subprocess)
- Update CONTRIBUTING.md with 'Workflow Choice: Legacy vs. v3 Plan
Lifecycle' section documenting the incompatibility and migration path
ISSUES CLOSED: #1560
