refactor(web): open produced files through the Host, not over HTTP

Scope decision: previews for a browser that is not on the Host machine are
not supported. With that settled, host.openPath answers the supported case
completely — a file:// document in a real browser has full page capabilities
and no reach into /api — and the HTTP serving this branch had built answered
only the unsupported one.

Removed: the /f route and its listener, the workspace-file URL shape,
ApiProxy.workspaceRootOf, ConnectionHandle.fileUrl, and the port published
into the index page.

Kept, and finished:
- the produced-files row a turn ends with, derived from mutation locations;
- the path link now reads as a link at rest, not only on hover — the reported
  "I can't open what it made" was this, sitting on a working capability;
- the Host opener prefers the default BROWSER for .html/.htm/.xhtml/.svg, so
  a developer who binds .html to an editor still gets a rendered page
  (macOS via the LaunchServices https handler, Linux via $BROWSER, every
  failure falling back to the default application).

The retired designs and their measurements stay in the Agent Note, including
why same-origin serving was unsafe and why the sandbox that fixed it broke
the pages invisibly.

packages/host/apiproxy/tests/native-path-opener.spec.ts

@@ -80,3 +80,96 @@ describe('native path opener', () => {
     })
   })
 })
+
+describe('browser-renderable documents', () => {
+  const LS_PLIST = `{
+    LSHandlers = (
+        {
+            LSHandlerPreferredVersions =             {
+                LSHandlerRoleAll = "-";
+            };
+            LSHandlerRoleAll = "com.google.chrome";
+            LSHandlerURLScheme = https;
+        }
+    );
+}`
+
+  it('opens a page with the default browser rather than the .html handler on darwin', async () => {
+    const calls: { command: string; args: readonly string[] }[] = []
+    const run = async (command: string, args: readonly string[]) => {
+      calls.push({ command, args })
+      return { stdout: command === 'defaults' ? LS_PLIST : '', stderr: '' }
+    }
+    await openNativePath('/w/page.html', new AbortController().signal, { platform: 'darwin', run })
+    // A developer who bound .html to an editor still gets a rendered page.
+    expect(calls.map(c => [c.command, ...c.args])).toEqual([
+      ['defaults', 'read', 'com.apple.LaunchServices/com.apple.launchservices.secure'],
+      ['open', '-b', 'com.google.chrome', '/w/page.html'],
+    ])
+  })
+
+  it('leaves every other document to the default application', async () => {
+    const calls: string[][] = []
+    const run = async (command: string, args: readonly string[]) => {
+      calls.push([command, ...args])
+      return { stdout: '', stderr: '' }
+    }
+    await openNativePath('/w/report.md', new AbortController().signal, { platform: 'darwin', run })
+    // No LaunchServices read at all: markdown is not a browser document.
+    expect(calls).toEqual([['open', '/w/report.md']])
+  })
+
+  it('falls back to the default application when no browser can be named', async () => {
+    // LaunchServices has no https record (a fresh account), so the system's
+    // own content-type choice is the best answer available.
+    const calls: string[][] = []
+    const run = async (command: string, args: readonly string[]) => {
+      calls.push([command, ...args])
+      if (command === 'defaults') throw new Error('domain not found')
+      return { stdout: '', stderr: '' }
+    }
+    await openNativePath('/w/page.html', new AbortController().signal, { platform: 'darwin', run })
+    expect(calls).toEqual([
+      ['defaults', 'read', 'com.apple.LaunchServices/com.apple.launchservices.secure'],
+      ['open', '/w/page.html'],
+    ])
+
+    // A record without an https handler is the same answer.
+    const bare: string[][] = []
+    await openNativePath('/w/page.html', new AbortController().signal, {
+      platform: 'darwin',
+      run: async (command, args) => {
+        bare.push([command, ...args])
+        return { stdout: '{ LSHandlers = ( ); }', stderr: '' }
+      },
+    })
+    expect(bare[1]).toEqual(['open', '/w/page.html'])
+  })
+
+  it('honors $BROWSER on linux and leaves windows to its association', async () => {
+    const linux: string[][] = []
+    await openNativePath('/w/page.html', new AbortController().signal, {
+      platform: 'linux',
+      env: { BROWSER: 'firefox' },
+      run: async (command, args) => { linux.push([command, ...args]); return { stdout: '', stderr: '' } },
+    })
+    expect(linux).toEqual([['firefox', '/w/page.html']])
+
+    // Unset $BROWSER: xdg-open's association is the fallback.
+    const bare: string[][] = []
+    await openNativePath('/w/page.html', new AbortController().signal, {
+      platform: 'linux',
+      env: {},
+      run: async (command, args) => { bare.push([command, ...args]); return { stdout: '', stderr: '' } },
+    })
+    expect(bare).toEqual([['xdg-open', '/w/page.html']])
+
+    // Windows names no browser without the UserChoice registry.
+    const win: string[][] = []
+    await openNativePath('C:\\w\\page.html', new AbortController().signal, {
+      platform: 'win32',
+      run: async (command, args) => { win.push([command, ...args]); return { stdout: '', stderr: '' } },
+    })
+    expect(win[0]?.[0]).toBe('powershell.exe')
+  })
+})