[codex] polish session list and header follow-ups

[gaius-codius]

Summary

• polish session list and session header metadata presentation
• add targeted coverage for session list, session header, and flavor utility behavior
• follow up with header simplification, machine label wiring, and related CSS cleanup
• ignore local planning docs under docs/superpowers/plans/

Why

• improve session glanceability and metadata consistency between list and header
• fix theme-related styling issues in flavor text and thinking indicators
• simplify header metadata and carry machine labels into the sessions page flow

Impact

• users get clearer session metadata in the list and header
• styling is more theme-aware and consistent
• developers get stronger focused tests around the touched UI utilities/components

Validation

• bun typecheck
• cd web && bunx vitest run src/components/SessionList.test.tsx src/components/SessionHeader.test.tsx src/lib/agentFlavorUtils.test.ts

Notes

• full web test suite still has existing unrelated React.act is not a function failures outside this PR scope
• branch is currently behind main; plan to sync/rebase before final review/merge