feat(cli): top-level --format / CBM_CLI_FORMAT default for tools (follow-up from #1867)

[Lowpower]

Split out of #1867 per @DeusData's request

Split out from #1867 (secondary finding there: search_graph / trace_path / detect_changes emit the compact tree by default when stdout is not a TTY), per the maintainer's invitation to give it its own discussion.

Current state (verified on v0.10.8) — better than my original report assumed

• Tools that accept a format argument (notably search_graph / trace_path / detect_changes) already support --format json, and <tool> --help documents it (e.g. search_graph --help: "Response encoding. tree (default) … json: the SAME tree model as structured JSON")
• The README CLI section documents both layers: tool-level --format json (payload JSON) vs outer --json (full MCP envelope)
• The tree default is deliberate, and I understand why: the primary non-TTY consumers of this CLI are AI agents and bridges, and the compact tree is token-optimized for exactly that audience

So this is not a bug report — it is a small UX/discoverability suggestion.

Remaining friction

1. --format json is per-tool-call only. A script or bridge invoking several tools must pass it on every single invocation; there is no way to set it once for a session.
2. The top-level cli --help doesn't mention it. You have to run <tool> --help for each tool to discover the flag. The README covers it, but --help is where habitual CLI users look first.

Suggestion — pick whichever direction you prefer; happy to submit a PR

• (a) Top-level flag: cli [--format tree|json] <tool> ... acting as a default for tools that accept format; an explicit per-tool --format still wins.
• (b) Env var: CBM_CLI_FORMAT=tree|json, useful for bridges that spawn the binary from generated glue code without full control over argv on every call.
• (c) Docs-only: add one line to the top-level cli usage string pointing at tool-level --format json.

I'd lean towards (a) + (b): both are fully backward compatible, and they deliberately do not flip the default on non-TTY stdout — auto-switching would silently change the output that model-facing bridges and agents see, which I assume is the audience the tree default was designed for.

I can implement this myself if you agree on a direction. From reading the source, the change looks localized:

• run_cli() in src/main.c — merge a top-level/env-provided format into args_json before tool dispatch (per-tool explicit value still takes precedence)
• tests/test_cli.c — precedence tests (top-level vs per-tool vs unset)
• CLI_USAGE string + README CLI section
• Per CONTRIBUTING (pre-commit hooks, scripts/lint.sh, DCO Signed-off-by trailer)

[DeusData]

Thank you for splitting this out and for clearly distinguishing the compact payload format from the outer MCP envelope. I checked the current CLI_USAGE in src/main.c:657-663: it describes --json, but does not point readers to tool-level --format json.

Let's take the help-only improvement first: add that pointer to top-level cli --help, keeping the distinction between payload JSON and --json's raw MCP result envelope explicit. A focused PR for that is welcome; please preserve the current output defaults and cover the help text in the existing CLI tests.

A session-wide flag or environment default needs a separate decision about supported tools and precedence. This first step does not approve that API extension, but it addresses the discoverability gap without changing existing invocations. Thank you for offering to contribute it.