feat: add shell completions and help commands

[alvinreal]

Adds shell completion generation and enhanced help commands for discoverability.

New CLI Options

Flag	Description
--completions <shell>	Generate shell completions (bash, zsh, fish, powershell, elvish)
--formats	Enhanced format listing with tabular layout
--functions	List all built-in mapping functions with signatures

Shell Completions

Uses clap_complete to generate completions for all major shells:

# Install bash completions
morph --completions bash > ~/.local/share/bash-completion/completions/morph

# Install zsh completions
morph --completions zsh > ~/.zsh/completions/_morph

# Install fish completions
morph --completions fish > ~/.config/fish/completions/morph.fish

Enhanced --formats Output

Now shows a formatted table:

Supported formats:

  FORMAT          EXTENSIONS   CAPABILITIES
  ──────          ──────────   ────────────
  JSON            json         read, write
  JSONL           jsonl, ndjson read, write
  ...

--functions Output

Lists all 30+ built-in functions organized by category:

• String: lower, upper, trim, replace, contains, split, join, etc.
• Type conversion: to_int, to_float, to_string, to_bool, type_of
• Math: abs, min, max, floor, ceil, round
• Null/existence: is_null, is_array, coalesce, default
• Collection: keys, values, unique, first, last, sum, group_by
• Conditional: if

Tests

6 integration tests:

• Bash/Zsh/Fish completion generation
• Unknown shell error handling
• --formats table output with all formats
• --functions output with categories and signatures

Fixes #30