feat(html-view): open network access for rendered pages (owner decision)

Owner decision 2026-10-13 (#547): a rendered .html page behaves like a
file opened from disk in a browser. It may load scripts, styles, fonts,
images and media from any host and make fetch, XHR, WebSocket, beacon
and form requests to any host.

The /html-view Content-Security-Policy is therefore the `sandbox`
directive alone (HTML_VIEW_CSP, now exported and documented): the same
sandbox flags as before, never `allow-same-origin`, and no fetch
directive. It replaces Codeman's own `default-src 'self'` policy on these
responses. Everything that confines a page stays: the opaque origin (no
Codeman cookies or document, API writes with `Origin: null` refused, API
reads not readable cross-origin), and the capability serving only the
page's own folder under every path, symlink, dot, extension and
confinement rule.

A new test builds the route behind registerSecurityHeaders, as in
production, and pins the exact header: one `sandbox` directive with the
current flags, no `allow-same-origin`, no `-src` directive. The security
architecture, architecture invariants, API reference, CLAUDE.md and the
Working With Files wiki page now say what a page can do (reach the
network, read its own folder) and cannot do (Codeman's origin, cookies,
API, other folders), and that this is the owner's decision.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

docs/wiki/Working-With-Files.md

@@ -106,7 +106,15 @@ was never what kept anything secret; the path guard is. Types outside the list (
 
 An `.html` file opens as a rendered page by default, with its relative CSS, scripts, images and
 data files. The page is served from a separate `/html-view/...` address in a sandbox with no
-origin of its own, so its scripts can never read Codeman's page, cookies or API. The **Page**
+origin of its own, so its scripts can never read Codeman's page, cookies or API.
+
+By the project owner's decision, a rendered page has the same network access as a file you open
+from disk in your browser. It **can** load scripts, styles, fonts, images and media from
+anywhere on the internet (a dashboard that pulls Chart.js from a CDN works), send requests to
+any host, and read the files in its own folder. It **cannot** act as Codeman: it has no access
+to Codeman's page, cookies or API, and no access to any folder other than its own. Since a page
+can send what it can read to any host, open HTML only from sources you trust, as you would on
+your own desktop. The **Page**
 pill in the viewer header switches to the source view (where **Edit** works as for any text
 file), and the viewer falls back to source on its own when a page cannot be rendered: a remote
 (SSH) case, an HTML file inside a hidden folder, or one sitting directly in a broad folder such