Now-section sweep: back-nav + lint migration

[That1Drifter]

Two small unrelated chores from the top of the TODO, bundled.

1. Back-to-scenarios navigation (65f5904)

Adds an "← all scenarios" link in the play screen header and a "Pick another scenario →" CTA below the rendered debrief output. Previously a user who finished a scenario could not start another one without editing the URL or hitting browser back — a broken flow disguised as polish.

2. Migrate `pnpm lint` from `next lint` to ESLint CLI (`3ed0ac2`)

`next lint` is deprecated in Next 15 and, when run non-interactively, drops into an ESLint setup prompt that ALSO mutates `tsconfig.json` (adds `allowJs: true`, reformats) and `next-env.d.ts` as side effects. We had to revert those mutations manually after every deploy. This bites every CI run too.

• Replaces the lint script with a flat `eslint.config.mjs` using `FlatCompat` to extend `next/core-web-vitals` + `next/typescript`
• Adds `eslint`, `@eslint/eslintrc`, `eslint-config-next` as devDeps
• Fixes two pre-existing `import/no-anonymous-default-export` warnings the broken lint was hiding (postcss.config.mjs and the new eslint.config.mjs both now assign to a named const before exporting — preferred over an eslint-disable comment)
• Verified tsconfig.json and next-env.d.ts are byte-identical to before

Test plan

• `pnpm typecheck` — clean across 6 packages
• `pnpm test` — all suites passing
• `pnpm lint` — exit 0, zero warnings, runs non-interactively
• `tsconfig.json` and `next-env.d.ts` unchanged after lint
• Smoke the back-nav on staging — click through play → debrief → "Pick another scenario" → picker

[That1Drifter]

Smoke test on staging passed. Drove the full flow with browse:

1. Loaded /play/support-triage — "← all scenarios" link rendered in the header (data-testid=back-to-scenarios)
2. Clicked it — landed on / picker, all 6 scenarios visible
3. Re-loaded the play page, ran a turn, clicked "debrief", waited for the narrative to render
4. data-testid=pick-another-scenario CTA appeared below the debrief output as expected
5. Clicked it — landed on / picker, screenshot confirmed

Both nav points working end-to-end.