feat(report): daily narrative on every today surface (Phase 4 M-A2)

[rojae]

Summary

Wires the LLM-written today, in a paragraph decoration onto every
today-shaped surface — the studio /today page, the CLI fluxmirror today text report, and the HTML day card all now lead with a
2-3 sentence narrative pulled from the existing M-A1 synthesis pipeline.

Pieces

DTO + helpers (fluxmirror-core)

• TodayData gains a narrative: DailyNarrative field with a sane
  Default so older callers and the JSON wire shape stay invariant.
• DailyNarrative carries paragraph + source (Llm | Heuristic)
  • optional model + cost_usd + cache_hit.
• report::data::build_daily_summary_input(today) returns the JSON
  context object that fills every {placeholder} in
  prompts/daily.txt plus richer keys for future prompt revisions
  (lifecycle hint, top files, agents, shell signature buckets,
  primary languages from file extensions).
• report::ai_narrative::heuristic_paragraph(today) is the always-on
  deterministic fallback. Properties: never empty, never panics on
  degenerate input, byte-stable for the same TodayData.

Synthesis wrapper (fluxmirror-ai)

• New module daily_narrative with
  synthesise_daily(store, config, today) -> DailyNarrative.
  Lives in fluxmirror-ai for the same reason session_intent and
  anomaly_story do — synthesise() depends on budget / cache /
  provider modules; pushing the wrapper down to core would
  re-introduce the cycle.
• Every error leg drops into heuristic_paragraph: provider=off,
  no writable store, empty window, budget hit, network error, empty
  completion text. The narrative field is therefore always
  populated regardless of provider state.

Surface wiring

• CLI today.rs — pulls TodayData via core::data::collect_today,
  populates narrative via synthesise_daily, prepends a
  ## Narrative section above the existing stats. Italic
  _(estimate)_ footnote when source = Heuristic.
• CLI html_day.rs — render_day_card now takes
  Option<&DailyNarrative>; injects a styled
  <section class="narrative"> at the top of the summary block plus
  the same estimate footnote.
• Studio api/today.rs — overlays narrative via
  fluxmirror_ai::synthesise_daily before responding.
• Frontend Today.tsx + Home.tsx — render the paragraph in a
  small panel at the top of each page with an estimate pill when
  source = 'heuristic'. Home also links the panel to /today.

Test plan

• cargo test --workspace --no-fail-fast
• bash scripts/test-rust-hook.sh
• bash scripts/check-report-commands.sh
• bash scripts/build-manifests.sh --check
• bash scripts/check-extension-shape.sh
• pnpm install --frozen-lockfile && pnpm build in studio-web/
• New tests:
  • crates/fluxmirror-core/tests/daily_narrative.rs — heuristic
    paragraph non-empty for busy + empty fixtures, snapshot pin,
    build_daily_summary_input key contract, zero-reads ratio
    fallback to "n/a".
  • crates/fluxmirror-studio/tests/api_today_narrative.rs —
    fixture DB + provider=off returns
    narrative.source = "heuristic" with non-empty paragraph;
    empty DB still emits the heuristic empty-window paragraph.
  • crates/fluxmirror-cli/src/cmd/report/html_day.rs — two new
    unit tests pin the narrative section + estimate footnote
    markup.