feat(web): order dashboard specs by created_at, newest first

[protonspy]

What

The web dashboard's spec list was ordered alphabetically by feature name. Order it by spec.json created_at descending instead, so the most recently created spec sits at the top of the panel.

All three surfaces (API GET /api/overview, TUI, CLI) go through session.LoadOverview, so they stay consistent — the change lives in one place.

Why

created_at is the true creation instant, stamped once at csdd spec init (time.Now().UTC().Format("2006-01-02T15:04:05Z")) and never rewritten. It is an RFC3339 UTC string (always …Z, fixed width), so a lexical string compare is chronological — no parsing needed. Filesystem mtime was rejected as a source: it means "last modified", changes on every edit, and is reset by git clone/checkout.

How

In LoadOverview, after building the spec cards, sort.Slice by CreatedAt descending. Equal created_at — and unreadable specs, whose created_at is empty (sorts last) — fall back to feature-name ascending for a stable, total order.

Tests

• TestOverviewSpecsSortedByCreatedAtDesc — three specs whose dates diverge from alphabetical order (alpha=Jan, beta=Jun, gamma=Mar → beta, gamma, alpha); catches any regression to name-sorting.
• TestOverviewSpecsCreatedAtTieBreaksByName — equal-created_at tie and the empty/unreadable case in one fixture.
• Existing TestOverview updated to assert newest-first ordering.

code-reviewer ran: no Blockers (APPROVE WITH NITS).

Verification

- go test -race ./internal/session/   ✅  83.8% coverage
- go test -race ./... (pre-push gate)  ✅  all packages green
- go vet ./internal/session/           ✅
- gofmt -l (touched files)             ✅  clean
- make build → ./csdd                  ✅

🤖 Generated with Claude Code