feat(templates): add TemplateLoader for context-aware template evaluation

[ghshhf]

Summary

This PR adds a dedicated TemplateLoader (new file core/templates/loader.go) that discovers and caches .tmpl files on disk, and wires it into the existing Evaluator so callers can enumerate available templates.

This is the second in a series of split PRs (per maintainer feedback). See #10317 for the overall discussion.

Changes

• New file: core/templates/loader.go

  • TemplateLoader type with ListTemplates / Resolve / Invalidate
  • Suffix-based filtering (only .tmpl files)
  • In-memory cache with filesystem re-scan on invalidate

• Modified: core/templates/evaluator.go

  • Evaluator now owns a TemplateLoader (created in NewEvaluator)
  • Expose loader via Evaluator.TemplateLoader() for UI / editor use
  • Separates template discovery from model loading (follow-up to the "Split ModelLoader and TemplateLoader" TODO)

Why Separate?

This is a self-contained, non-controversial change:

• No P2P/auth/cache complexity
• No dead code
• Clear use case (template enumeration for model editor UI)
• New file with clean separation of concerns

Follow-up PRs (from #10317)

1. ✅ PR 1: Config YAML endpoints (feat(config): add GET /api/models/config-yaml/:name endpoint #10318)
2. ✅ PR 2 (this PR): Template/context pipeline
3. PR 3: Metrics/monitoring additions
4. PR 4: MCP HTTP API routes/client hardening
5. PR 5: Reasoning parser (with tests)
6. PR 6: Distributed cache (with tests, cache invalidation docs)
7. PR 7: P2P node snapshot + ReplaceNodes (design doc, cancellation safety)
8. PR 8: Realtime ephemeral key (HMAC tests, userID binding)

Testing

• ListTemplates() returns all .tmpl files in the templates directory
• Resolve("chatml") finds chatml.tmpl on disk
• Invalidate() forces re-scan on next ListTemplates() call
• Existing template evaluation still works (no regression)

Related Issues

Part of #10317