Announce diagnostics endpoint and agent skills in dev-server startup output

[ryansolid]

Summary

• With diagnostics: true, the dev server startup block gains two lines after Vite's URLs: the /__solid/diagnostics endpoint (with its POST method vocabulary) and the paths of the agent skill documents shipped in node_modules (@solidjs/diagnostics agent loops, solid-js reactivity repair guide).
• Startup output is the one channel agents reliably read even in projects with no AGENTS.md — this is the discovery path for existing apps and ports, complementing the scaffold-level pointers added to the solid-v2 templates (solid-v2: enable agent diagnostics and add AGENTS.md to all templates templates#283).
• Dev-serve only; no output change for builds, preview, or when the option is off.

Verified live against a scaffolded app:

  ➜  Local:   http://localhost:3011/
  ➜  Network: use --host to expose
  ➜  Solid diagnostics: http://localhost:3011/__solid/diagnostics (GET status; POST {"method":"begin"|"end"|"whyDidRun"|"costs"})
  ➜  Agent skills: node_modules/@solidjs/diagnostics/skills/agent-loops/SKILL.md, node_modules/solid-js/skills/reactivity-diagnostics/SKILL.md

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Latest commit: 5c50681

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
@solidjs/vite-plugin	Patch

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commit: 5c50681