docs(quickstart): link Onchain AI Garage Hermes tutorials playlist (#20192)

* revert(gateway): remove stale-code self-check and auto-restart

Removes the _detect_stale_code / _trigger_stale_code_restart mechanism
introduced in #17648 and iterated in #19740. On every incoming message
the gateway compared the boot-time git HEAD SHA to the current SHA on
disk, and if they differed it would reply with

    Gateway code was updated in the background --
    restarting this gateway so your next message runs
    on the new code. Please retry in a moment.

and then kick off a graceful restart. This is unwanted behaviour:
users who run a long-lived gateway and do their own ad-hoc git
operations on the checkout end up with their chat interrupted and
the current message dropped every time HEAD moves, with no way to
opt out.

If an operator really needs the old protection against stale
sys.modules after "hermes update", the SIGKILL-survivor sweep in
hermes update (hermes_cli/main.py, also tagged #17648) already
handles the supervisor-respawn case on its own.

Removed:
  gateway/run.py:
    - _STALE_CODE_SENTINELS, _GIT_SHA_CACHE_TTL_SECS
    - _read_git_head_sha(), _compute_repo_mtime() module helpers
    - class-level _boot_wall_time / _boot_repo_mtime / _boot_git_sha /
      _stale_code_restart_triggered defaults
    - __init__ boot-snapshot block (_boot_*, _cached_current_sha*,
      _repo_root_for_staleness, _stale_code_notified)
    - _current_git_sha_cached(), _detect_stale_code(),
      _trigger_stale_code_restart() methods
    - stale-code check + user-facing restart notice at the top of
      _handle_message()
  tests/gateway/test_stale_code_self_check.py (deleted, 412 lines)

No new logic added. Zero remaining references to any removed
symbol. Gateway test suite passes the same 4589 tests it passed
before; the 3 pre-existing unrelated failures (discord free-channel,
feishu bot admission, teams typing) are unchanged by this commit.

* docs(quickstart): link Onchain AI Garage Hermes tutorials playlist

Adds a 'Prefer to watch?' tip callout near the top of the quickstart page pointing to @OnchainAIGarage's Hermes Agent Tutorials + Use Cases playlist, which includes a Masterclass series covering install, setup, and basic commands.

* docs(quickstart): embed Masterclass video in Prefer to watch section

Swaps the plain-link tip callout for an inline responsive YouTube embed of the Hermes Agent Masterclass (R3YOGfTBcQg) plus a kept link to the full Onchain AI Garage tutorials playlist.

website/docs/getting-started/quickstart.md

@@ -8,6 +8,21 @@ description: "Your first conversation with Hermes Agent — from install to chat
 
 This guide gets you from zero to a working Hermes setup that survives real use. Install, choose a provider, verify a working chat, and know exactly what to do when something breaks.
 
+## Prefer to watch?
+
+**Onchain AI Garage** put together a Masterclass walkthrough of installation, setup, and basic commands — a good companion to this page if you'd rather follow along on video. For more, see the full [Hermes Agent Tutorials & Use Cases](https://www.youtube.com/channel/UCqB1bhMwGsW-yefBxYwFCCg) playlist.
+
+<div style={{position: 'relative', paddingBottom: '56.25%', height: 0, overflow: 'hidden', maxWidth: '100%', marginBottom: '1.5rem'}}>
+  <iframe
+    style={{position: 'absolute', top: 0, left: 0, width: '100%', height: '100%'}}
+    src="https://www.youtube-nocookie.com/embed/R3YOGfTBcQg"
+    title="Hermes Agent Masterclass: Installation, Setup, Basic Commands"
+    frameBorder="0"
+    allow="accelerometer; clipboard-write; encrypted-media; gyroscope; picture-in-picture"
+    allowFullScreen
+  ></iframe>
+</div>
+
 ## Who this is for
 
 - Brand new and want the shortest path to a working setup