feat(files): preview text files from outside the workspace, and stop routing them at a viewer that cannot read them

A .json/.log/.yaml/code path outside the session workspace was refused as an
unsupported type, and clicking one in the terminal made it worse: text goes to
the log viewer, which spawns `tail -f` and allows only the workspace, /var/log
and ~/logs, so it answered "Path must be within working directory or allowed
log directories" while the same path clicked in the response viewer previewed
fine. Two surfaces, two answers, for a file the session can already cat.

- TEXT_ATTACHMENT_EXTENSIONS IS EDITABLE_EXTENSIONS (config/file-editing.ts),
  not a second curated list that would drift from it. The rule reads: if the
  viewer would open a file for editing inside the workspace, the same file
  outside it can be read. The suffix was never the confidentiality gate here,
  the path guard is (sensitive-file blocklist, /root and /etc trees, realpath
  before the check), and it still runs on every registration.
- Widening what can be READ must not widen what can RUN. html/htm join svg in
  serveRawFile's download-only branch, so markup is never served with a
  renderable type on our own origin; other text goes out as inert
  text/plain; charset=utf-8 with nosniff, matching what the path picker does.
  The preview reads through fetch(), which ignores the disposition, so a
  clicked .html still shows its source.
- ~/.codeman*/state.json joins isSensitivePath. It persists
  SessionState.envOverrides and the env allowlist admits key-shaped names
  (GEMINI_API_KEY, CLAUDE_CODE_*), so it can hold a live credential. Same
  treatment as hook-secret and users.json, and the rest of the tree stays
  attachable.
- The terminal sends an out-of-workspace path to the preview instead of the log
  viewer. In-workspace text keeps the tail viewer, which is the point of it, and
  file-stream-manager's allowlist is untouched: no `tail -f` on arbitrary host
  paths.
- The by-id text preview is bounded like the workspace one: a Range request for
  the first 512KB (a real partial read, not a discarded 50MB download) plus a
  500-line cap, with the footer saying so.

Verified on an isolated instance: a 1.1MB external log opens in ~1.8s showing
500 lines with "showing first 500 lines" in the footer; json, yaml and code
preview; an .html carrying a script tag renders as source and does not execute;
.svg is still refused; a terminal click on an external .yaml opens the preview
with no log viewer and no attachment card; an in-workspace .log still opens the
streaming tail viewer.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

src/attachment-registry.ts

@@ -11,6 +11,7 @@ import { realpathSync } from 'node:fs';
 import fs from 'node:fs/promises';
 import { basename, extname, isAbsolute } from 'node:path';
 import { isBlockedAttachmentPath, loadAttachmentGuardConfig } from './config/attachment-guard.js';
+import { EDITABLE_EXTENSIONS } from './config/file-editing.js';
 import { validateSessionFilePath } from './web/route-helpers.js';
 import type { AttachmentDetectedEvent, AttachmentDetectedType } from './types.js';
 
@@ -34,6 +35,22 @@ export const AUDIO_ATTACHMENT_EXTENSIONS: ReadonlySet<string> = new Set([
   'opus',
 ]);
 
+/**
+ * Plain-text extensions, REUSING the File Viewer's edit-mode allowlist rather
+ * than curating a second list that would drift from it. The rule reads: if the
+ * viewer would open that file for editing inside the workspace, the same file
+ * outside it can be read here. `svg` and `env` are absent from that list by
+ * design and stay absent here.
+ *
+ * Why widen at all: the agent in the session can already `cat` any of these,
+ * and every path-shaped surface (the picker, the workspace viewer) can already
+ * show them. Refusing a `.log` an agent just wrote to `/tmp` bought no
+ * confidentiality, it only made the click fail. The confidentiality gate is the
+ * path guard that still runs on every registration (sensitive-file blocklist,
+ * `/root` and `/etc` trees, realpath before the check), not the file's suffix.
+ */
+export const TEXT_ATTACHMENT_EXTENSIONS: ReadonlySet<string> = EDITABLE_EXTENSIONS;
+
 const SUPPORTED_ATTACHMENT_EXTENSIONS = new Set([
   'png',
   'jpg',
@@ -47,6 +64,7 @@ const SUPPORTED_ATTACHMENT_EXTENSIONS = new Set([
   'txt',
   ...VIDEO_ATTACHMENT_EXTENSIONS,
   ...AUDIO_ATTACHMENT_EXTENSIONS,
+  ...TEXT_ATTACHMENT_EXTENSIONS,
 ]);
 
 export type AttachmentSource = 'detected' | 'external';
@@ -135,7 +153,9 @@ export function getAttachmentType(extension: string): AttachmentDetectedType {
   if (normalized === 'pdf') return 'pdf';
   if (normalized === 'pptx') return 'presentation';
   if (normalized === 'md') return 'markdown';
-  if (normalized === 'txt') return 'text';
+  // Everything else in the text family reads as text, including code and
+  // config: the card and the preview both treat it as a plain-text file.
+  if (normalized === 'txt' || TEXT_ATTACHMENT_EXTENSIONS.has(normalized)) return 'text';
   return 'document';
 }