feat(builtins): add template command (non-standard, Mustache/Handlebars-lite)

[chaliy]

Summary

A non-standard builtin for lightweight template rendering. envsubst (#551) handles simple $VAR substitution, but real config generation needs conditionals, loops, and defaults. This fills the gap between envsubst and a full template engine.

Proposed Syntax

template [OPTIONS] [template-file] [< stdin]

Variables come from environment + shell variables. Additional vars via -v key=value.

Supported Template Syntax (Mustache-lite)

Syntax	Description
{{VAR}}	Variable substitution (HTML-safe by default)
{{{VAR}}}	Raw substitution (no escaping)
{{VAR|default}}	Default value if unset
{{#items}}...{{/items}}	Section (loop over array or truthy check)
{{^items}}...{{/items}}	Inverted section (render if falsy/empty)
{{! comment }}	Comment (stripped from output)

Use Cases

# Simple config generation
export HOST=localhost PORT=8080
template < nginx.tmpl > nginx.conf

# With extra variables
template -v env=production -v replicas=3 < k8s.tmpl

# Inline template
echo 'Hello {{USER}}, you are in {{PWD}}' | template

# Conditional sections
cat <<'TMPL' | ENABLE_SSL=true template
server {
    listen {{PORT|80}};
    {{#ENABLE_SSL}}
    ssl_certificate /etc/ssl/cert.pem;
    {{/ENABLE_SSL}}
}
TMPL

# Loop over space-separated values
export SERVICES="web api worker"
cat <<'TMPL' | template
{{#SERVICES}}
- {{.}}
{{/SERVICES}}
TMPL

llm_hint()

Some("template: Mustache-lite templates. `echo '{{VAR|default}}' | template`. Supports sections {{#X}}...{{/X}}, defaults {{V|d}}, comments {{! }}.")

Implementation Notes

• Variables sourced from interpreter env + shell variables
• Arrays (space-separated strings) iterable via {{#VAR}}...{{/VAR}}
• {{VAR|default}} syntax for defaults is critical — most templates need it
• NOT Mustache-spec compliant (no partials, no lambdas) — intentionally simple
• Exit 1 if template has syntax errors, 0 otherwise (undefined vars → empty string)

[chaliy]

Documentation Requirements

Since this is a non-standard (invented) builtin, it must be documented in two places:

1. README.md — Add to the "Built-in Commands" table under a new Extensions category row (non-standard bashkit-specific commands, clearly marked as extensions)
2. crates/bashkit/docs/extensions.md — Create or update a dedicated extensions guide covering all invented builtins, with short descriptions, examples, and rationale for why each exists

This ensures users discover these commands and understand they are bashkit-specific, not standard Bash.

[chaliy]

Resolved in PR #585 or #588