docs(command-development): document bash ! prefix for pre-execution syntax (#55)

## Summary

- Document the `!`command`` syntax required for bash pre-execution in
actual command files
- Explain that pre-execution happens before Claude sees the command
- Clarify why skill examples intentionally omit `!` (would execute when
loaded into context)
- Update reference file examples to use correct `!` prefix for
copy-paste use

## Problem

Fixes #46

The command-development skill didn't document the `!` prefix syntax
required for bash pre-execution in actual command files. Users following
the examples would write `\`git status\`` instead of `!\`git status\``,
resulting in non-functional commands.

## Solution

1. Added "Syntax: The `!` Prefix" section to SKILL.md explaining:
   - The exact `!`command`` syntax
   - How pre-execution works (before Claude sees the prompt)
   - Example showing input and expanded output

2. Added "Why Skill Examples Omit `!`" section explaining the
intentional omission in skill documentation

3. Updated all bash examples in `plugin-features-reference.md` to use
the `!` prefix, since users copy these into actual command files

### Alternatives Considered

- Adding HTML comments to examples: Less visible, easily overlooked
- Creating separate "for copying" examples: Would duplicate content

## Changes

- `plugins/plugin-dev/skills/command-development/SKILL.md`: Added 40
lines documenting the `!` prefix syntax
-
`plugins/plugin-dev/skills/command-development/references/plugin-features-reference.md`:
Updated 21 bash examples to use `!` prefix

## Testing

- [x] Markdown linting passes
- [x] Documentation is clear and complete
- [x] Examples show correct syntax for actual command files

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

---------

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

Author: sjnims

plugins/plugin-dev/skills/command-development/README.md

@@ -334,14 +334,54 @@ Ensure:
 
 Commands can execute bash commands inline to dynamically gather context before Claude processes the command. This is useful for including repository state, environment information, or project-specific context.
 
+### Syntax: The `!` Prefix
+
+In actual command files, use the `!` prefix before backticks for pre-execution:
+
+```markdown
+Current branch: !`git branch --show-current`
+Files changed: !`git diff --name-only`
+Environment: !`echo $NODE_ENV`
+```
+
+**How it works:**
+
+1. Before Claude sees the command, Claude Code executes `!`command`` blocks
+2. The bash output replaces the entire `!`command`` expression
+3. Claude receives the expanded prompt with actual values
+
+**Example expansion:**
+
+Command file contains:
+
+```markdown
+Review the !`git diff --name-only | wc -l | tr -d ' '` changed files on branch !`git branch --show-current`.
+```
+
+Claude receives (after pre-execution):
+
+```markdown
+Review the 3 changed files on branch feature/add-auth.
+```
+
+### Why Skill Examples Omit `!`
+
+Examples in skill documentation use plain backticks without `!`:
+
+```markdown
+Files changed: `git diff --name-only`
+```
+
+This is intentional. When skill content loads into Claude's context, `!`command`` would actually execute. Skill examples show the conceptual pattern; add the `!` prefix when writing actual command files.
+
 **When to use:**
 
 - Include dynamic context (git status, environment vars, etc.)
 - Gather project/repository state
 - Build context-aware workflows
 
 **Implementation details:**
-For complete syntax, examples, and best practices, see `references/plugin-features-reference.md` section on bash execution. The reference includes the exact syntax and multiple working examples to avoid execution issues
+For advanced patterns, environment-specific configurations, and plugin integration, see `references/plugin-features-reference.md`
 
 ## Command Organization