[good first issue] Add --pretty flag for indented JSON output (distinct from --human)

[abhicris]

Context

Every subcommand in src/commands.rs calls output::emit(mode, label, value) with the result, and src/output.rs chooses between Mode::Json and Mode::Human. But the global --human flag only enables pretty-printing — it does not pretty-print JSON itself. Agents consuming JSON from stdout and humans debugging in a terminal both benefit from the jq-style indented variant.

Scope

• In src/output.rs:
  • Add Mode::JsonPretty variant.
  • Update emit:
    • Mode::Json → serde_json::to_string(value) (compact — existing behavior).
    • Mode::JsonPretty → serde_json::to_string_pretty(value).
    • Mode::Human → existing pretty colored output.
• In src/main.rs:
  • Add --pretty global flag. Mutually exclusive with --human.
  • If --pretty is set, use Mode::JsonPretty.
• Update README's example outputs to include --pretty examples.

Acceptance criteria

• arbitrum-cli block latest → single-line JSON (unchanged).
• arbitrum-cli block latest --pretty → indented JSON.
• arbitrum-cli block latest --human → colored table (unchanged).
• --human --pretty exits non-zero with a clear clap error.
• All existing tests pass.

Reference: src/output.rs::emit is 30 lines; PR #2 (feat: agent-deposit command + GitHub Actions CI) for CI/test expectations.

Estimated effort

S (1–2 hours)

— kcolbchain / Abhishek Krishna