security(paste-image): harden against 7 findings from PR #84 review (#90)

Hardens `/api/sessions/:id/paste-image` against the seven findings flagged in the dismissed security review on #84. Each commit addresses one finding.

- LOW: Collision-free filenames (`paste-${ts}-${rand4}${ext}`)
- MED: Symlink check on image dir (`lstat` + non-recursive mkdir + `O_EXCL|O_NOFOLLOW`)
- MED: Magic-byte validation (PNG/JPEG/GIF/WebP/BMP)
- HIGH: CSRF protection (Origin/Referer match req.host; non-browser clients send `X-Codeman-CSRF`)
- MED: Swap hand-rolled multipart parser to @fastify/multipart with `limits: { fileSize: 10MB, files: 1, fields: 4 }`
- MED: Rate limit (30/min per IP+session) + hourly GC of `paste-*` files older than 7d
- LOW: Use `terminal.paste(text)` instead of `sendInput(text)` so bracketed-paste markers survive

Co-authored-by: Aamer Akhter <aakhter@gmail.com>

src/web/paste-image-gc.ts

@@ -0,0 +1,69 @@
+/**
+ * @fileoverview Periodic GC for paste-image files.
+ *
+ * Without cleanup, /api/sessions/:id/paste-image accumulates files indefinitely
+ * under {workingDir}/.claude-images/. The route only triggers cleanup on
+ * killMux=true session deletion, so long-lived sessions can fill disk under
+ * heavy pasting. This sweeper bounds disk use by deleting `paste-*` files
+ * older than MAX_AGE_MS from each live session's image dir on an interval.
+ *
+ * Conservative defaults — only files matching the `paste-` prefix are
+ * considered, and we lstat (not stat) so a planted symlink cannot escape the
+ * image dir.
+ */
+import fs from 'node:fs/promises';
+import { join } from 'node:path';
+import type { SessionPort } from './ports/index.js';
+
+const MAX_AGE_MS = 7 * 24 * 60 * 60 * 1000; // 7 days
+const SWEEP_INTERVAL_MS = 60 * 60 * 1000; // 1 hour
+const INITIAL_DELAY_MS = 30 * 1000; // 30s after startup
+
+export async function sweepPasteImagesOnce(
+  ctx: Pick<SessionPort, 'sessions'>,
+  now: number = Date.now()
+): Promise<{ scanned: number; deleted: number }> {
+  const cutoff = now - MAX_AGE_MS;
+  let scanned = 0;
+  let deleted = 0;
+  for (const session of ctx.sessions.values()) {
+    const dir = join(session.workingDir, '.claude-images');
+    let entries: string[];
+    try {
+      entries = await fs.readdir(dir);
+    } catch {
+      continue; // dir absent — nothing to do
+    }
+    for (const name of entries) {
+      if (!name.startsWith('paste-')) continue;
+      const p = join(dir, name);
+      scanned += 1;
+      try {
+        const st = await fs.lstat(p);
+        if (!st.isFile()) continue;
+        if (st.mtimeMs < cutoff) {
+          await fs.unlink(p);
+          deleted += 1;
+        }
+      } catch {
+        // best-effort: skip permission/race errors silently
+      }
+    }
+  }
+  return { scanned, deleted };
+}
+
+export function startPasteImageGc(ctx: Pick<SessionPort, 'sessions'>): () => void {
+  const initial = setTimeout(() => {
+    void sweepPasteImagesOnce(ctx);
+  }, INITIAL_DELAY_MS);
+  const interval = setInterval(() => {
+    void sweepPasteImagesOnce(ctx);
+  }, SWEEP_INTERVAL_MS);
+  if (typeof initial.unref === 'function') initial.unref();
+  if (typeof interval.unref === 'function') interval.unref();
+  return (): void => {
+    clearTimeout(initial);
+    clearInterval(interval);
+  };
+}