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/plugins/codex-harness.md

@@ -693,11 +693,11 @@ turn for that agent must be a Codex-supported OpenAI model.
 
 ## Related
 
-- [Agent Harness Plugins](/plugins/sdk-agent-harness)
+- [Agent harness plugins](/plugins/sdk-agent-harness)
 - [Agent runtimes](/concepts/agent-runtimes)
-- [Model Providers](/concepts/model-providers)
+- [Model providers](/concepts/model-providers)
 - [OpenAI provider](/providers/openai)
 - [Status](/cli/status)
 - [Plugin hooks](/plugins/hooks)
-- [Configuration Reference](/gateway/configuration-reference)
+- [Configuration reference](/gateway/configuration-reference)
 - [Testing](/help/testing-live#live-codex-app-server-harness-smoke)