COD-37 add server-side attachment pipeline (registry, magic-link, path guard)

Adds the foundation for serving local files to the browser as live external
attachments with a stable id, so requests never carry arbitrary absolute paths.

- attachment-registry: in-memory, session-scoped registry. registerExternalAttachment
  validates an absolute path, resolves symlinks, enforces the path guard, and mints
  an `att_<uuid>` id; records are cleared when the session is removed.
- attachment path guard: a configurable blocklist (secret locations + /root,/etc
  trees, extendable via attachmentBlockedPaths / CODEMAN_ATTACHMENT_BLOCKED_PATHS)
  plus an optional, default-off workspace-confinement mode. Shares one
  sensitive-path blocklist (web/sensitive-path.ts) with /api/download, which is
  refactored to use the extracted module instead of an inline copy.
- terminal magic links: the session scans output for codeman://attach?path=... and
  emits `attachmentRequested`; the web server registers the file and broadcasts an
  `attachment:detected` SSE event. `codeman attach <path>` (CLI) prints the magic
  link or POSTs directly when a session id is known.
- image watcher: detects png/pdf/docx/pptx dropped into a session's working dir and
  emits `attachment:detected`.
- routes: POST /api/sessions/:id/attachments (register) and
  GET /api/sessions/:id/attachments/:attachmentId/raw (serve), both re-checking the
  guard before streaming.

Document previews/thumbnails and the attachment-history drawer build on this
foundation and land separately.

Verified: tsc --noEmit, lint, format, frontend-syntax, full test:ci (2846 passed),
and a server boot smoke (/api/status 200).

src/attachment-magic.ts

@@ -0,0 +1,35 @@
+/**
+ * @fileoverview Parses terminal magic links that request attachment cards.
+ */
+
+import { isAbsolute } from 'node:path';
+import { isSupportedAttachmentExtension } from './attachment-registry.js';
+
+const MAGIC_LINK_RE = /codeman:\/\/attach\?([^\s<>"']+)/g;
+
+export function parseAttachmentMagicLinks(data: string): string[] {
+  const results: string[] = [];
+  const seen = new Set<string>();
+
+  for (const match of data.matchAll(MAGIC_LINK_RE)) {
+    const query = trimTrailingPunctuation(match[1] || '');
+    try {
+      const params = new URLSearchParams(query);
+      const filePath = params.get('path');
+      if (!filePath || !isAbsolute(filePath)) continue;
+      const extension = filePath.split('.').pop()?.toLowerCase() || '';
+      if (!isSupportedAttachmentExtension(extension)) continue;
+      if (seen.has(filePath)) continue;
+      seen.add(filePath);
+      results.push(filePath);
+    } catch {
+      // Ignore malformed terminal text. Magic links are advisory.
+    }
+  }
+
+  return results;
+}
+
+function trimTrailingPunctuation(value: string): string {
+  return value.replace(/[),.;:]+$/g, '');
+}