[codex] Add backlog-first project core

[screenleon]

Summary

• Add first-class backlog_items backend storage, APIs, labels, origin validation, and urgent priority support.
• Make ProjectDetail backlog-first: Backlog is the default primary tab, Workspace is relabelled as Planning, and backlog rows gain clearer summary/status behavior.
• Fix sandbox-hostile remote model probe tests by replacing httptest.NewServer provider calls with an in-memory RoundTripper.
• Record the backlog-first direction in DECISIONS.md, docs/backlog-first-plan.md, docs/data-model.md, and docs/api-surface.md.

Review Fixes

• Critic/risk/security/architecture review flagged blocked backlog commit, cross-project origin references, committed item edits drifting from created tasks, filtered urgent counts, nested interactive row controls, and API doc drift.
• Fixed blocked backlog commit rejection, same-project origin validation, committed task snapshot immutability for title/description/priority, explicit edit/commit controls, project-level focus counts, and API docs envelope/placement.
• B3 generated-planning materialization into durable backlog_items is documented as follow-up scope; this PR covers B1/B2 and the backlog UX cleanup before connector auto-implementation.

Validation

• GOCACHE=/tmp/anpm-go-cache make pre-pr
• npm test -- --run BacklogTab ProjectDetail
• GOCACHE=/tmp/anpm-go-cache TEST_DATABASE_URL=sqlite go test -timeout 240s ./internal/handlers
• git diff --check
• Pre-push backend SQLite test hook passed during git push -u origin codex/backlog-first-core.

Notes

• A sandboxed make pre-pr attempt initially failed because the default Go build cache under ~/.cache/go-build is read-only in this environment. The passing run used GOCACHE=/tmp/anpm-go-cache as shown above.