Experiment with adding autocomplete

[ptrus]

Fixes: #4

Wanted to experiment with adding autocomplete a bit, feels quite nice.

Note: this is heavily AI-generated code.

How to test:

  # 1. Build the CLI
  go build -o oasis .

  # 2. Enable completions for current session (use your shell: bash, zsh, fish, or powershell)
  source <(./oasis completion zsh)

   # 3. Test
  ./oasis <TAB>
  ./oasis account transfer --network <TAB>
  ./oasis account transfer 100 <TAB>

Examples:

   # === BASIC COMMAND/SUBCOMMAND COMPLETION ===
  ./oasis <TAB>                              # lists: account, addressbook, contract, network, paratime, rofl, tx, wallet
  ./oasis account <TAB>                      # lists: allow, burn, delegate, deposit, show, transfer, undelegate, withdraw, entity...
  ./oasis network <TAB>                      # lists: add, governance, remove, set-chain-context, set-default, set-rpc, show, status...
  ./oasis network governance <TAB>           # lists: cast-vote, create, list, show

  # === FLAG COMPLETION (--flag) ===
  ./oasis account transfer --<TAB>           # lists all flags: --network, --paratime, --account, --offline, --gas-limit...
  ./oasis network show --<TAB>               # lists: --network, --height, --format...

  # === FLAG VALUE COMPLETION ===
  ./oasis account transfer --network <TAB>   # lists: mainnet, testnet
  ./oasis account transfer --paratime <TAB>  # lists paratimes for default network
  ./oasis account transfer --account <TAB>   # lists wallet names

  # === POSITIONAL ARGUMENT COMPLETION ===
  ./oasis wallet remove <TAB>                # wallet names only
  ./oasis wallet show <TAB>                  # wallet names only
  ./oasis addressbook remove <TAB>           # addressbook names only
  ./oasis account show <TAB>                 # wallet + addressbook names
  ./oasis account transfer 100 <TAB>         # wallet + addressbook names (at position 1)
  ./oasis network remove <TAB>               # network names

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