[codex] Add docs markdown fetch instructions (#1597)

## Summary

- Add Markdown-fetch instructions to the hosted `skill.hexclave.com`
prompt docs section.
- Document `llms.txt`, `llms-full.txt`, and per-page `.md` URLs for
agents fetching docs directly.

## Validation

- Verified live docs behavior with `curl`: `.md` page URLs return
Markdown, and `llms.txt` / `llms-full.txt` are reachable.
- Ran `git diff --check`.
- `pnpm -C apps/skills typecheck` could not run because this worktree
does not have `node_modules` installed (`tsc: command not found`).

<!-- This is an auto-generated description by cubic. -->
---
## Summary by cubic
Add explicit Markdown fetch instructions to the skill-site prompt so
agents can pull docs from the live navigation.
Includes `https://docs.hexclave.com/llms.txt` discovery,
`https://docs.hexclave.com/llms-full.txt` bundle, and per-page `.md` URL
examples on `https://docs.hexclave.com`.

<sup>Written for commit 7d5de50179.
Summary will update on new commits.</sup>

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<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->

## Summary by CodeRabbit

* **Documentation**
* Enhanced documentation endpoint guidance with specific HTTPS URLs for
retrieving documentation as Markdown format, including discovery
endpoints and single-page access patterns.

<!-- end of auto-generated comment: release notes by coderabbit.ai -->

packages/shared/src/ai/unified-prompts/skill-site-prompt.ts

@@ -19,7 +19,10 @@ export function buildSkillSitePrompt(docsIndexPromptValue = docsIndexPrompt) {
 
     ## Docs
 
-    The full docs sidebar — generated from the live navigation. Fetch any of these directly:
+    The full docs sidebar — generated from the live navigation. Fetch any of these directly.
+
+    To retrieve docs as Markdown, use these endpoints:
+    - For a specific docs page, append \`.md\` to the canonical docs URL. For example, fetch \`https://docs.hexclave.com/guides/getting-started/setup.md\` for the Markdown version of \`https://docs.hexclave.com/guides/getting-started/setup\`.
 
     ${docsIndexPromptValue}