docs(skill): add well-known skills discovery endpoint (#135)

## Summary
- Add `/.well-known/skills/index.json` endpoint for agent skills
auto-discovery
- Update `generate-skill.ts` to output SKILL.md to both plugin and
well-known paths
- Follows the [Cloudflare Agent Skills Discovery
RFC](https://github.com/cloudflare/agent-skills-discovery-rfc) pattern

## Test plan
- [x] Verify `bun run docs:dev` serves `/.well-known/skills/index.json`
with valid JSON
- [x] Verify `/.well-known/skills/sentry-cli/SKILL.md` returns the skill
file
- [x] Verify `bun run build` includes files in dist output
- [ ] After deploy, test with `npx skills add https://cli.sentry.dev`

🤖 Generated with [Claude Code](https://claude.com/claude-code)

---------

Co-authored-by: Claude Opus 4.5 <noreply@anthropic.com>

Author: sergical

docs/public/.well-known/skills/index.json

@@ -0,0 +1,9 @@
+{
+  "skills": [
+    {
+      "name": "sentry-cli",
+      "description": "Guide for using the Sentry CLI to interact with Sentry from the command line. Use when the user asks about viewing issues, events, projects, organizations, making API calls, or authenticating with Sentry via CLI.",
+      "files": ["SKILL.md"]
+    }
+  ]
+}

docs/public/.well-known/skills/sentry-cli/SKILL.md

@@ -0,0 +1 @@
+../../../../../plugins/sentry-cli/skills/sentry-cli/SKILL.md