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/web/server.ts

@@ -60,6 +60,7 @@ import {
   type SubagentToolResult,
 } from '../subagent-watcher.js';
 import { imageWatcher } from '../image-watcher.js';
+import { attachmentRegistry, registerExternalAttachment } from '../attachment-registry.js';
 import { TranscriptWatcher } from '../transcript-watcher.js';
 import { TeamWatcher } from '../team-watcher.js';
 import { TunnelManager } from '../tunnel-manager.js';
@@ -110,6 +111,7 @@ import {
   type PersistedRespawnConfig,
   type NiceConfig,
   type ImageDetectedEvent,
+  type AttachmentDetectedEvent,
   DEFAULT_NICE_CONFIG,
 } from '../types.js';
 import {
@@ -248,6 +250,7 @@ export class WebServer extends EventEmitter {
   } | null = null;
   private imageWatcherHandlers: {
     detected: (event: ImageDetectedEvent) => void;
+    attachmentDetected: (event: AttachmentDetectedEvent) => void;
     error: (error: Error, sessionId?: string) => void;
   } | null = null;
   private tunnelManager: TunnelManager = new TunnelManager();
@@ -436,12 +439,15 @@ export class WebServer extends EventEmitter {
     // Store handlers for cleanup on shutdown
     this.imageWatcherHandlers = {
       detected: (event: ImageDetectedEvent) => this.broadcast(SseEvent.ImageDetected, event),
+      attachmentDetected: (event: AttachmentDetectedEvent) =>
+        this.broadcast(SseEvent.AttachmentDetected, { ...event, source: event.source || 'detected' }),
       error: (error: Error, sessionId?: string) => {
         console.error(`[ImageWatcher] Error${sessionId ? ` for ${sessionId}` : ''}:`, error.message);
       },
     };
 
     imageWatcher.on('image:detected', this.imageWatcherHandlers.detected);
+    imageWatcher.on('attachment:detected', this.imageWatcherHandlers.attachmentDetected);
     imageWatcher.on('image:error', this.imageWatcherHandlers.error);
   }
 
@@ -451,6 +457,7 @@ export class WebServer extends EventEmitter {
   private cleanupImageWatcherListeners(): void {
     if (this.imageWatcherHandlers) {
       imageWatcher.off('image:detected', this.imageWatcherHandlers.detected);
+      imageWatcher.off('attachment:detected', this.imageWatcherHandlers.attachmentDetected);
       imageWatcher.off('image:error', this.imageWatcherHandlers.error);
       this.imageWatcherHandlers = null;
     }
@@ -1067,6 +1074,8 @@ export class WebServer extends EventEmitter {
       session.removeAllListeners();
       // Close any active file streams for this session
       fileStreamManager.closeSessionStreams(sessionId);
+      // Drop live external attachment registrations for this session
+      attachmentRegistry.clearSession(sessionId);
       // Stop watching for images in this session's directory
       imageWatcher.unwatchSession(sessionId);
       // Clean up pasted images directory for this session
@@ -1252,9 +1261,23 @@ export class WebServer extends EventEmitter {
         }
       },
       getStore: () => this.store,
+      registerAttachment: (id: string, filePath: string) => this.registerAttachment(id, filePath),
     };
   }
 
+  /**
+   * Register a terminal-requested external file as a live attachment and
+   * broadcast it. Triggered by the session's `attachmentRequested` event
+   * (codeman://attach magic links). Registration enforces the COD-53
+   * attachment-guard policy.
+   */
+  private async registerAttachment(sessionId: string, filePath: string): Promise<void> {
+    const session = this.sessions.get(sessionId);
+    if (!session) return;
+    const event = await registerExternalAttachment(sessionId, filePath, { sessionWorkingDir: session.workingDir });
+    this.broadcast(SseEvent.AttachmentDetected, event);
+  }
+
   private setupRespawnListeners(sessionId: string, controller: RespawnController): void {
     wireRespawnListeners(sessionId, controller, this.buildRespawnWiringDeps());
   }