Snapshots YAML template content in a new `plan_template_versions` table
whenever a plan is created from a template. Plans reference their version
via `template_version_id` so existing plans are unaffected by future
template file edits.
Key changes:
- Migration 0006: create `plan_template_versions` table (slug, version,
name, content JSON, content_hash SHA-256); add nullable FK
`template_version_id` to `agent_plans`
- Model `PlanTemplateVersion`: `findOrCreateFromTemplate()` deduplicates
identical content by hash; `historyFor()` returns versions newest-first
- `AgentPlan`: add `template_version_id` fillable and `templateVersion()`
relationship
- `PlanTemplateService::createPlan()`: snapshot raw template before
variable substitution; store version id and version number in metadata;
add `getVersionHistory()` and `getVersion()` public methods
- Tests: `TemplateVersionManagementTest` covering model behaviour, plan
creation snapshotting, deduplication, history ordering, and service
methods
Closes#35
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
