feat(scripted_tool): help builtin for runtime schema introspection (#529)

## Summary

- Add `HelpBuiltin` providing runtime tool schema introspection inside
ScriptedTool scripts
- `help --list` — list all tool names + descriptions
- `help <tool>` — human-readable usage with flags
- `help <tool> --json` — machine-readable JSON schema (pipeable to jq)
- Add `ScriptedToolBuilder::compact_prompt(bool)` — when enabled,
`system_prompt()` emits only names + one-liners and defers full schemas
to `help`
- 8 new tests covering all help modes, compact/full prompt, and jq
piping

## Test plan

- [x] `cargo test --lib scripted_tool` — all 40 tests pass
- [x] `cargo fmt --check` — clean
- [x] `cargo clippy --all-targets --all-features -- -D warnings` — clean

Closes #519

Author: chaliy