chore(uploads): remove the broken upload page and deprecate /api/screenshots

The tunnel Upload URL page (upload.html) has been broken since the response envelope landed in 458fb81c: it reads j.filename and j.files while the server answers { success, data: { filename } } and { success, data: { files } }, so every upload reported "Saved: undefined" and the recent list stayed empty. Nothing else reads ~/.codeman/screenshots/, and handing a file to an agent goes through POST /api/sessions/:id/paste-image into the session's own workspace, so the page, its Settings row, the suffix branch of the tunnel row helper (now folded into its one caller) and the Upload URL i18n key go.

The three /api/screenshots routes keep working unchanged and log one deprecation warning per process on first use, naming paste-image as the replacement. Per docs/versioning-policy.md they are removed in a later MAJOR, after at least one MINOR release that carries the warning; the docs, CLAUDE.md and the multi-user plan say so.

Static caching: with upload.html gone no HTML is served by @fastify/static any more (every page has its own no-cache route; on a built tree only the precompressed index.html.gz artifact is reachable, as application/gzip, and nothing requests it). The .html branch of setHeaders was therefore dead and goes with the test that fetched upload.html to reach it; a comment now says a new static HTML page needs its own route. The index.html no-cache assertion on the route stays.

CLAUDE.md

@@ -127,7 +127,7 @@ Codeman is a Claude Code session manager with web interface and autonomous Ralph
 
 **Code style**: Prettier (`singleQuote: true`, `printWidth: 120`, `trailingComma: "es5"`) — config lives in the **`"prettier"` key of `package.json`**, not a `.prettierrc` (keeps the repo root short; editors read it natively). `.prettierignore` stays at the root because Prettier resolves it relative to cwd. ESLint flat config (`config/eslint.config.js`) allows `no-console`, warns on `@typescript-eslint/no-explicit-any`. Ignores: `app.js`, `scripts/**/*.mjs`, `src/web/public/vendor/**`, `scripts/remotion/**`.
 
-**Prettier scope is deliberately narrow.** `npm run format` globs only `src/**/*.ts` and `src/web/public/**` (`lint` only `src/**/*.ts`), and `.prettierignore` then exempts most of `src/web/public/*.js` (app.js, styles.css, **mobile.css**, index.html, upload.html, and 15 hand-formatted modules) plus `CLAUDE.md`. Those files are hand-formatted by design; `npm run check:public-assets` and `check:frontend-syntax` are what guard them (NUL bytes + JS syntax), not Prettier. Do not "fix" a file by adding it back to Prettier's scope.
+**Prettier scope is deliberately narrow.** `npm run format` globs only `src/**/*.ts` and `src/web/public/**` (`lint` only `src/**/*.ts`), and `.prettierignore` then exempts most of `src/web/public/*.js` (app.js, styles.css, **mobile.css**, index.html, and 15 hand-formatted modules) plus `CLAUDE.md`. Those files are hand-formatted by design; `npm run check:public-assets` and `check:frontend-syntax` are what guard them (NUL bytes + JS syntax), not Prettier. Do not "fix" a file by adding it back to Prettier's scope.
 
 ## Common Gotchas
 
@@ -487,7 +487,7 @@ curl -sk https://localhost:3000/api/subagents | jq # Background agents
 cat ~/.codeman/state.json | jq                     # Persisted state
 ```
 
-Mobile screenshots: `~/.codeman/screenshots/`, accessed via `GET/POST /api/screenshots`.
+Legacy screenshots (deprecated): `GET/POST /api/screenshots` still read and write `~/.codeman/screenshots/`, but the upload page that fed them is gone, they log a one-time deprecation warning, and they are removed in a later MAJOR, after at least one MINOR release that carries the warning (`docs/versioning-policy.md`). To hand a file to an agent use `POST /api/sessions/:id/paste-image`.
 
 ## Performance & Limits