feat: plan_snapshot field on ClaudeJob + architecture doc (#23)

* feat: add plan_snapshot field to ClaudeJob schema

Nullable String? column on claude_jobs captures the task's
implementation_plan at claim time — immutable baseline for drift detection.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

* docs: update ClaudeJob lifecycle with plan_snapshot

Document state machine snapshot capture/reset, plan_snapshot field
rationale, and drift-detection baseline semantics.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

* refactor: remove duplicate header labels on backlog page

Both the product H1 + description in the page header and the
"Product Backlog" panel-title in the PBI panel duplicated info
already visible in the NavBar. Removed both, keeping the right-aligned
action bars (activate/sprint/settings, plus filters/+PBI) intact.

PanelNavBar component is unchanged — Stories and Taken panels keep
their titles.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

---------

Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>

prisma/migrations/20260430171227_add_claude_job_plan_snapshot/migration.sql

@@ -0,0 +1,2 @@
+-- AlterTable
+ALTER TABLE "claude_jobs" ADD COLUMN     "plan_snapshot" TEXT;

prisma/schema.prisma

@@ -260,6 +260,7 @@ model ClaudeJob {
   claimed_at          DateTime?
   started_at          DateTime?
   finished_at         DateTime?
+  plan_snapshot       String?
   branch              String?
   summary             String?
   error               String?