feat(template): implement Go html/template engine

[zeroedin]

Summary

• Implement TemplateEngine interface using Go's html/template package
• Support {{ .page.title }} expressions, {{ if }}/{{ else }} conditionals, {{ range }} loops
• Support {{ block }}/{{ define }} layout inheritance with default content
• Raw HTML content injection via template.HTML conversion for the content field
• AddFilter registers functions into the template's FuncMap

Test results

Test group	Result
GoEngine Parse and Render	5/5
GoEngine Layout inheritance	2/2
Template package total	120/120

Test plan

• go test ./internal/template/... — 120/120 pass
• go vet ./... — clean
• No regressions in other packages

🤖 Generated with Claude Code

[zeroedin]

Code Review: feat(template): implement Go html/template engine

Clean, minimal implementation. 120/120 template tests pass, integration tests improved (7/16, up from 6/16 on main), go vet clean. The GoEngine is now fully functional for the test suite's expectations.

Summary

Area	Verdict
Parse / Render	✅ Correct — wraps html/template properly
AddFilter → FuncMap	✅ Correctly wires FilterFunc into the template's FuncMap
AddTag	⚠️ Silent no-op (same issue as PR #4)
markHTMLSafe	⚠️ Works but has design limitations — see inline
Test results	✅ Verified 120/120
Integration regressions	✅ None — net +1 pass over main

Overall this is good to merge. Two items worth addressing — one now (the markHTMLSafe depth issue) and one deferred (AddTag).

See inline comments for details.

[zeroedin]

All three review comments addressed in commit 8f55831:

1. markHTMLSafe recursion (line 56) — Fixed. Now recurses into nested map[string]interface{} and converts both content and summary fields to template.HTML at any depth.

2. AddTag silent no-op (line 39) — Fixed. AddTag now wraps TagFunc in a variadic func(args ...string) string and registers it in FuncMap, so Go template syntax like {{ youtube "id" }} works.

3. AddFilter ordering doc (line 30) — Fixed. Added doc comment noting that AddFilter must be called before Parse due to Go's html/template parse-time function binding.