fix: Scheduled autotitling can overwrite concurrent session metadata

[sam-saffron-jarvis]

What changed

• Persist scheduled autotitles through the existing generated-title-only store API instead of a full session-row update.
• Update the in-memory title fields only after persistence succeeds.
• Add a focused SQLite-backed regression test that captures a stale candidate, changes its manual name, pinned state, model, status, and goal, then verifies autotitling changes only generated-title fields.
• Assert that the command save path calls UpdateGeneratedTitle and never Update when the store supports the title-only capability.

Why this is high-value

The autotitle command loads all candidate sessions before making serial provider calls, so a later candidate can be stale for minutes. A full-row Update from that snapshot could silently revert user-visible changes made while the job was running, including a manual title, pin/status state, model selection, and goal. The title-only update removes that realistic race without changing title generation behavior.

Validation

• gofmt -w cmd/sessions_autotitle.go cmd/sessions_autotitle_test.go
• go build ./...
• go test ./cmd -run '^TestUpdateAutotitleDoesNotClobberConcurrentSessionMetadata$' -count=1
• go test ./... with an isolated HOME/XDG_CONFIG_HOME (passes; the normal agent home injects its global skill catalog into two unrelated cwd-skill tests)
• git diff --check