docs: the codegraph ui section, changelog entry and telemetry posture (CG-47)

A new user can now reach the viewer from the README alone: a "Read your
graph in the browser" section with a screenshot re-shot from the real
build, a step 5 in Get Started, a CLI Reference row, and the same
content as a docs-site guide.

- README: new section (what the three columns are, the options, the
  privacy posture), Contents entry, Get Started step 5, CLI row.
  Screenshot at assets/codegraph-ui-symbol-view.png, version-tagged ?v=1.
- CHANGELOG: an [Unreleased] New Features entry in the user-facing voice.
- codegraph help ui: mentions the `web` alias, says what the screen shows,
  and states that nothing is sent anywhere.
- TELEMETRY.md: the viewer has no telemetry of its own and makes no
  outbound connections; the only thing recorded is the command name in
  the daily rollup, which every off-switch already suppresses.
- site/: guides/viewer.md + sidebar entry, a `ui` section in the CLI
  reference, and a link from Next Steps.

TELEMETRY.md

@@ -65,6 +65,18 @@ And one of four events:
 Usage is **aggregated locally into daily totals** before anything is sent — there is no
 per-call event stream, and nothing is sent in real time.
 
+### The browser viewer sends nothing
+
+`codegraph ui` (the local viewer) has no telemetry of its own. The server it starts
+makes no outbound connections at all, and the page in your browser talks only to that
+server on `127.0.0.1`: nothing about the symbols you open, the searches you type, or the
+path you walk leaves your machine, and none of it is recorded anywhere. The only thing
+telemetry ever learns about the viewer is what it learns about every command: that a
+command named `ui` was run, once, on a day, in the daily `usage_rollup` above. The
+command never triggers a send of its own, and `codegraph telemetry off`,
+`CODEGRAPH_TELEMETRY=0`, or `DO_NOT_TRACK=1` switches off even that count, as it does
+everything else on this page.
+
 ## What is never collected
 
 - **No source code.** No file paths, file names, directory names, repository names or