feat(cli): session-wide format default (top-level --format / CBM_CLI_FORMAT) — follow-up from #2102

[Lowpower]

Context

Follow-up to #2102 / #2140.

Help-only discoverability ((c)) is done: top-level cli --help now points at tool-level --format json, and keeps payload JSON vs outer --json (MCP envelope) distinct.

In #2102, @DeusData noted that a session-wide flag or environment default needs a separate decision about supported tools and precedence, and that the help-only step did not approve that API extension. This issue is that decision request — discussion only; no PR until you say the shape is OK.

Proposal

Keep today’s defaults unchanged (including non-TTY: still no auto-switch to JSON).

• (a) Top-level flag: cli [--format tree|json] <tool> ... as a default for tools that already accept format; an explicit per-tool --format still wins.
• (b) Env var: CBM_CLI_FORMAT=tree|json for bridges that spawn the binary without full control over argv on every call.

I’d lean (a) + (b) if both are acceptable; either alone is also fine if you prefer a smaller surface.

Open questions for maintainers

1. Supported tools — apply only where format already exists today (search_graph / trace_path / detect_changes and any peers), or also refuse/warn when --format / CBM_CLI_FORMAT is set for a tool that has no format arg?
2. Precedence — suggestion:
   per-tool --format > top-level --format > CBM_CLI_FORMAT > built-in default (tree)
3. Invalid values — hard error vs ignore-and-default for unknown format / env values?
4. Docs — top-level CLI_USAGE + README CLI section only, or also each tool’s --help?

Non-goals

• Do not flip non-TTY stdout to JSON automatically.
• Do not change the meaning of outer --json (full MCP envelope).

Happy to implement once you pick a direction (or say neither). Happy to adjust the precedence / tool set to match your preference.