fix(attachments): harden registry + close magic-link injection vector

Security (MAJOR): the terminal-output codeman://attach scanner registered any
matching path server-side with no user confirmation and broadcast the rawUrl
over SSE. Terminal output is attacker-influenceable (a prompt-injected session
can print an arbitrary path), so on the default no-auth deployment this was an
arbitrary host-file (png/pdf/docx/pptx/md/txt) read primitive reachable by any
SSE client. Magic-link registration is now force-confined to the session
workspace (forceWorkspaceConfinement) regardless of the global confine setting;
deliberate cross-workspace attach still works through the explicit,
Origin-guarded POST /attachments route and 'codeman attach' (which POSTs
directly inside a managed session). Documented in security-architecture.md.

Regression (MAJOR): .png was rerouted from the image-popup path to
attachment:detected, which has no frontend consumer — silently breaking the
dropped/pasted-screenshot popup. PNG stays on image:detected; only pdf/docx/pptx
(which never had a popup) emit attachment:detected.

Also:
- raw route streams the freshly-resolved path, not the stored one, so a
  post-registration symlink swap can't redirect the stream (TOCTOU).
- 50MB cap on the attachment raw route, matching file-raw / download.
- per-session attachment registry cap (200) to bound the POST path.
- CLI reads creds via dataPath('.env'), honoring CODEMAN_INSTANCE.

Tests: forced-confinement reject/allow cases; PNG popup-path assertions updated.

src/image-watcher.ts

@@ -20,8 +20,12 @@ import { KeyedDebouncer } from './utils/index.js';
 // ========== Constants ==========
 
 /** Supported image file extensions (lowercase) */
-const IMAGE_POPUP_EXTENSIONS = new Set(['.jpg', '.jpeg', '.gif', '.webp', '.bmp', '.svg']);
-const ATTACHMENT_EXTENSIONS = new Set(['.png', '.pdf', '.docx', '.pptx']);
+// PNG stays on the image-popup path: it's the dominant screenshot format and the
+// frontend only wires the `image:detected` popup today. The attachment-card UI
+// that would consume `attachment:detected` for images is out of scope for this
+// PR, so routing PNG to it would silently break the dropped-screenshot popup.
+const IMAGE_POPUP_EXTENSIONS = new Set(['.png', '.jpg', '.jpeg', '.gif', '.webp', '.bmp', '.svg']);
+const ATTACHMENT_EXTENSIONS = new Set(['.pdf', '.docx', '.pptx']);
 const DETECTED_FILE_EXTENSIONS = new Set([...IMAGE_POPUP_EXTENSIONS, ...ATTACHMENT_EXTENSIONS]);
 
 /** Time to wait for file writes to stabilize (ms) */