(codex): index Codex sessions into the sidebar, search and viewer #821

Codex CLI support, step 2: index existing Codex sessions read-only (upstream 896783b, e71e6f0, the Codex title index from fe7fba0, viewer half of 966715d, final upstream/main:harnesses/codex.js; credit Ali Basiri). Depends on the harness port issue.

User-visible: Codex history in the sidebar, search and message viewer, with a CLI badge; Codex sessions count in heatmap, stats and ActivityWatch (decided by JB, 2026-10-28).

Keep (.ai/contexts/session-cache.md): transcript cwd trust (#385); a running session keeps its row; project remap ignores Codex rows; delete-session refuses Codex rows or deletes via sessionFile; continuation and subagent rollups ignore runtime='codex'. Scan and watcher ($CODEX_HOME/sessions) go through the fork's worker and writer. Audit path-safety for folder keys like codex/yyyy/mm/dd.

Tests: upstream's codex fixtures (id from file name, subagent rollouts skipped, response_item-only counting); worker scan over a fake CODEX_HOME.