docs: unify casing and replace path-as-text links across recent doc surfaces

Sweep recent (last ~5h) doc edits for two readability/uniformity issues:

- Replace 42 path-as-text links of the form '[/foo/bar](/foo/bar)' with
  descriptive labels derived from each target page's frontmatter title
  (e.g. '[Anthropic]', '[Token use and costs]', '[OpenAI-compatible
  endpoints]'). Affected files include gateway/troubleshooting,
  concepts/oauth, reference/session-management-compaction, and
  reference/transcript-hygiene.
- Sentence-case Title-Cased headings and link text in Related sections
  across codex-harness, model-providers, tools/plugin, sdk-runtime,
  sdk-setup, prompt-caching, ci, cli/config, google-meet, browser,
  rich-output-protocol, subagents, web/control-ui, while preserving
  brand and proper-noun capitalization (OpenAI, Codex, Chrome, Parallels,
  Z.AI, etc.).

docs/web/control-ui.md

@@ -318,7 +318,7 @@ Trusted-proxy note:
   device identity
 - this does **not** extend to node-role Control UI sessions
 - same-host loopback reverse proxies still do not satisfy trusted-proxy auth; see
-  [Trusted Proxy Auth](/gateway/trusted-proxy-auth)
+  [Trusted proxy auth](/gateway/trusted-proxy-auth)
 
 See [Tailscale](/gateway/tailscale) for HTTPS setup guidance.