refactor: standardize system keys with underscore prefix

Breaking changes:
- Rename `args:` frontmatter key to `_inputs:`
- Rename `env:` (object form) to `_env:` for process.env
- Template variables now require underscore prefix (e.g., {{ _name }})
  to prevent namespace collision with CLI flags like --model

New features:
- Add shell_escape filter (also aliased as `q`) for safe shell quoting
- All underscore-prefixed keys are now consistently treated as system keys

Template collision prevention:
- extractTemplateVars() now only returns underscore-prefixed variables
- {{ model }} no longer consumes --model CLI flag
- {{ _model }} is the new convention for template variables

Updated:
- schema.ts: _inputs and _env validation
- command.ts: expanded SYSTEM_KEYS set
- command-builder.ts: _env extraction, skip object values
- template.ts: shell_escape filter, underscore-only extraction
- All test files updated for new conventions
- CLAUDE.md and README.md documentation updated

Author: johnlindquist

.beads/beads.db

@@ -95,7 +95,8 @@ Commands are resolved in priority order:
 - `_stdin`: Auto-injected template variable containing piped input
 - `_1`, `_2`, etc.: Auto-injected positional CLI args (e.g., `md task.md "foo"` → `{{ _1 }}` = "foo")
 - `_args`: Auto-injected numbered list of all positional args
-- `env` (object form): Sets process.env before execution
+- `_inputs`: Named positional arguments to consume from CLI (e.g., `_inputs: [_message]`)
+- `_env`: Sets process.env before execution
 - `$1`, `$2`, etc.: Map positional args to flags
 - `_interactive`: Enable interactive mode (overrides print-mode defaults)
 - `_subcommand`: Prepend subcommand(s) to CLI args (e.g., `_subcommand: exec`)
@@ -112,7 +113,7 @@ dangerously-skip-permissions: true  # → --dangerously-skip-permissions
 add-dir:                     # → --add-dir ./src --add-dir ./tests
   - ./src
   - ./tests
-env:                         # Object form: sets process.env
+_env:                        # Sets process.env (underscore prefix = system key)
   API_KEY: secret
 ---
 ```

.packx_cache/cache.json

@@ -401,10 +401,12 @@ mdflow create-feature.claude.md --_feature_name "Auth"
 
 ### Environment Variables
 
+Use `_env` (underscore prefix) to set environment variables for the command:
+
 ```markdown
 # api-test.claude.md
 ---
-env:
+_env:
   API_URL: https://api.example.com
   DEBUG: "true"
 ---