Implement `render` subcommand and `internal/render` package

[mikkeldamsgaard]

Summary

Implement the render subcommand and internal/render package for template rendering with environment variable substitution and optional Go templates.

Requirements

• internal/render package:
  • envsubst mode: replace ${VAR} and $VAR patterns with environment variable values
  • Go text/template mode: full Go template support with env vars as data
  • Mode selection via --mode envsubst|gotemplate (default: envsubst)
• render subcommand:
  • --template flag: path to template file (required)
  • --output flag: relative output path within workdir (required)
  • --workdir flag: output directory (default /work)
  • Path traversal prevention via internal/safety
  • Create intermediate directories as needed

Acceptance Criteria

• internal/render/render.go with envsubst and Go template implementations
• internal/render/render_test.go with tests for: envsubst, Go templates, missing vars, empty input, special characters
• internal/cmd/render.go with full subcommand implementation
• internal/cmd/render_test.go with command-level tests
• Registered in cmd/initium/main.go
• docs/usage.md updated
• CHANGELOG.md updated
• All tests pass

[mikkeldamsgaard]

Implemented in commit 02cd94b. All acceptance criteria met:

• ✅ internal/render/render.go with envsubst and Go template implementations
• ✅ internal/render/render_test.go with 17 tests (envsubst: basic, missing vars, empty, no vars, empty value, special chars, multiline, adjacent, underscores, dollar-without-var; gotemplate: basic, missing var, empty, invalid syntax, conditional, special chars, envToMap)
• ✅ internal/cmd/render.go with full subcommand implementation
• ✅ internal/cmd/render_test.go with 11 command-level tests (no template, no output, invalid mode, envsubst, gotemplate, path traversal, missing template, nested output dir, invalid syntax, JSON output, empty template)
• ✅ Registered in cmd/initium/main.go
• ✅ docs/usage.md updated with render section
• ✅ CHANGELOG.md updated
• ✅ All tests pass (go test ./... → all ok)