feat(output): default to json instead of toon for non-TTY stdout (DX-5867)

[johnpmitsch]

Summary

Makes JSON the default output format when stdout is not a terminal (piped / scripted / agent invocations), replacing TOON. JSON is understood by virtually every downstream tool and parses cleanly, so it's the safer universal default for the piped path. TOON is newer and not as widely supported, so it's a poor implicit default.

TOON stays fully supported via --format toon and the config file. Only the default-when-unset changes: the CLI flag > config > built-in default ordering is unchanged, and the TTY default stays table.

The only behavioral edit is one branch in resolve_output_inner (src/context.rs). Everything else syncs docs to the new state: module docs, --format help, the embedded qn agent context guide, and the README (intro example, formats table, config defaults).

Test plan

• cargo test — all pass (117+ tests), including the updated default_is_json_when_stdout_is_not_a_tty and a new config_toon_overrides_non_tty_default
• cargo clippy --all-targets -- -D warnings — clean
• cargo fmt --check — clean
• cargo build --release — clean
• qn agent context and qn --help both report JSON as the non-TTY default

Closes DX-5867