Add one-click install/sign-in to sentry-triage setup gate (#2831)

The sentry-triage canvas's setup gate previously told users to ask
Copilot to install its optional `sentry` dependency and reload
extensions, and to run `sentry auth login` from a terminal. Both
flows were unreliable or high-friction in practice.

This adds real one-click buttons for both steps:

- sentryClient.mjs — installPackage() runs npm install (async, via
  execFile so the shared extension process's event loop isn't
  blocked) rooted at the file's own directory via import.meta.url, so
  the path is never guessed. loadFactory() falls back to importing
  the package's resolved entry file by absolute path when the bare
  import('sentry') fails, since Node caches a negative resolution for
  a bare specifier for the life of the process — while still
  distinguishing a genuinely missing package from a transitive-
  dependency defect so the latter isn't masked as "package missing."
  login() fails fast with a clear message when SENTRY_AUTH_TOKEN/
  SENTRY_TOKEN is active in the environment (which takes precedence
  over the OAuth login this button drives), and treats an empty/
  falsy auth.login() result as a failed sign-in instead of silently
  re-probing.
- preflight.mjs — installDependencies() runs the install and
  re-probes the connection; authenticate() classifies the new error
  codes for gate messaging.
- server.mjs / extension.mjs — wire POST /api/install-dependencies
  and POST /api/auth-login routes (through the existing CSRF/Host
  gate) to the new preflight functions, publishing refreshed
  connection state to the canvas.
- components/page.mjs / styles.mjs — the install and sign-in
  buttons, with loading/success/failure states and accessible live-
  region status updates; a live multi-org <select> dropdown that
  rebuilds in place once org discovery completes post-signin,
  without requiring a canvas reopen.
- README.md — documents the one-click flow; manual npm install /
  sentry auth login kept as a fallback.

Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>

extensions/sentry-triage/README.md

@@ -40,17 +40,11 @@ that drafts a fix pull request.
 
 - **Node.js 22 or newer.**
 - The GitHub Copilot app canvas / UI-extensions experiment enabled.
-- A Sentry sign-in. This canvas reads issues through the
+- **A Sentry sign-in.** This canvas reads issues through the
   [Sentry CLI](https://cli.sentry.dev) in library mode — there is no MCP server
-  to configure. After installing the dependency (see **Install** below), sign in
-  once with the package-local CLI, run from the extension folder:
-
-  ```sh
-  npx sentry auth login
-  ```
-
-  This stores an OAuth credential the canvas auto-detects. For non-interactive
-  environments, export a token instead:
+  to configure. The setup gate walks you through installing the dependency and
+  signing in with one-click buttons (see **Install** below); no terminal
+  required. For non-interactive environments, you can instead export a token:
 
   ```sh
   export SENTRY_AUTH_TOKEN=<your-token>
@@ -67,30 +61,17 @@ repository at `.github/extensions/sentry-triage/` for project scope.
 
 This canvas depends on the [`sentry`](https://cli.sentry.dev) npm package at
 runtime, which isn't bundled with the extension source. If it's missing, the canvas
-still **opens** and shows a setup gate explaining what to do instead of crashing.
+still **opens** and shows a setup gate — click its **Install dependencies** button
+and the extension runs `npm install` in its own directory (wherever it's actually
+installed, so there's no path to guess). Once installed, if you aren't signed in
+yet the gate shows a **Sign in with Sentry** button that opens your browser to
+approve access and returns automatically — no terminal step required. The gate
+clears once both finish; no reload or Copilot involvement needed.
 
-### Let Copilot set it up (recommended)
+### Or set it up manually
 
-Because the canvas runs inside GitHub Copilot, the agent can install the dependency
-for you. Paste this into Copilot:
-
-> Locate the loaded `sentry-triage` canvas extension folder — the directory that
-> contains its `package.json` — run `npm install` there, then reload extensions.
-
-(That folder is `~/.copilot/extensions/sentry-triage/` for user scope,
-`.github/extensions/sentry-triage/` for project scope, or, if you installed the
-published plugin, `com.github.copilot/extensions/sentry-triage` inside the
-installed plugin.)
-
-Then finish the one interactive step yourself — sign in so Copilot never handles a
-raw secret. Run this from the same extension folder (`npx` resolves the CLI the
-local `npm install` just placed in `node_modules`):
-
-```sh
-npx sentry auth login
-```
-
-### Or install it manually
+If you'd rather not use the buttons (or `npm`/a browser isn't available to the
+extension process), you can run the same steps yourself:
 
 ```sh
 # User scope