feat: contextual examples in command help text

Summary

Static help text for commands lacks per-pallet or per-chain examples. Users must guess at syntax, especially for complex calls like XCM transfers or nested enum types.

Motivation

dot hash already has good inline examples (lines 52-69). Other commands should follow this pattern, and ideally provide metadata-aware examples when chain metadata is cached.

Proposed Changes

Phase 1: Enriched static help

• Add comprehensive example blocks to dot tx, dot query, dot inspect help text
• Include examples for common patterns: simple transfers, enum arguments, nested structs, Option types
• Show examples of --dry-run, --output json, and other flags in context

Phase 2 (optional): Metadata-aware help

• When metadata is cached, dot tx Balances --help could list available calls with their signatures
• dot tx Balances.transfer_allow_death --help could show the full argument schema

Dependencies

• Benefits from inspect listing calls (Issue #79 or equivalent)
• Benefits from #54 (tx browsing)

Priority

P3 — Nice-to-have after core UX is solid. Size M.

Start by comparing the existing dot hash help examples at lines 52-69 with the help entry points for dot tx, dot query, and dot inspect. Review Issues #79 and #54 for dependent command capabilities. Done means the specified commands show contextual examples for transfers, enum and nested arguments, optional values, and relevant flags; metadata-aware help is an optional follow-up.