feat(scripted_tool): built-in `help <tool> --json` for runtime schema introspection

[chaliy]

Summary

Add a built-in help command to ScriptedTool that lets LLMs query tool schemas at runtime, instead of requiring all schema details in the upfront system_prompt().

Motivation

Inspired by Cloudflare Code Mode MCP: their search() tool lets LLMs explore API specs at runtime rather than loading everything into context. Currently system_prompt() emits full schemas for all registered tools. For small sets (4-10) this is fine; for large sets (50+) it blows up the context window.

Runtime introspection lets system_prompt() emit only tool names + one-liners, deferring full schemas to on-demand queries within the bash script.

Design

Built-in help command

Registered automatically alongside user tools in ToolBuiltinAdapter:

# Human-readable (default)
help get_user
# get_user - Fetch user by ID
# Usage: get_user --id <integer> [--include-orders <boolean>]

# Machine-readable JSON (for jq pipelines)
help get_user --json
# {"name":"get_user","description":"Fetch user by ID","input_schema":{...}}

# List all tools
help --list
# get_user     Fetch user by ID
# list_orders  List orders for user

# Discover enum values
help get_user --json | jq '.input_schema.properties.role.enum'
# ["admin", "user", "guest"]

Implementation

1. Add a HelpBuiltin struct in execute.rs that holds Vec<RegisteredTool> (clone of tool defs)
2. Register it as builtin help alongside user tools during execute()
3. Flag parsing: --json for JSON output, --list for summary, bare name for man-page style
4. system_prompt() gains a new tip: "Use help <tool> --json for full parameter details"

system_prompt() changes

For tools with schemas, system_prompt() can optionally emit a compact form:

## Available tool commands

- `get_user`: Fetch user by ID (use `help get_user` for params)
- `list_orders`: List orders for user (use `help list_orders` for params)

Add a ScriptedToolBuilder::compact_prompt(bool) option to control this. Default: false (backward compat).

Acceptance criteria

• help <tool> prints human-readable usage
• help <tool> --json prints JSON schema to stdout (pipeable to jq)
• help --list prints all tool names + descriptions
• compact_prompt(true) omits full schemas from system_prompt(), adds help tip
• Tests for all modes
• Update spec 014

Effort

Small — schema data already exists in ToolDef.input_schema. Just needs a new builtin.