docs(contract): codify partial-codec policy in §4

§4 enumerated three codecs (markdown, json, csv) without saying
whether implementing all three was required. The Status table after
PR #10 acknowledges partial-codec exporters (crono and liftoff ship
markdown+json, not csv), but a reader who jumps straight to §4 still
gets the "all three required" impression.

Add explicit language:

- CLIs MAY implement a subset.
- markdown and json are both REQUIRED — markdown serves human
  terminal use, json serves agent/script use, omitting either leaves
  a use case unserved.
- csv is recommended but optional.
- compat.Runner.SupportedFormats (per PR #10) is the framework
  affordance that lets a partial-codec exporter wire the bundle.

This is mergeable standalone — the policy stands whether or not
PR #10's harness affordance has merged. Without #10 the policy is
purely documentary; with #10 the policy becomes machine-verifiable.

Co-Authored-By: Claude Opus 4.7 (1M context) <[email protected]>

Co-authored-by: Orca <[email protected]>

Author: DTTerastar

CONTRACT.md

@@ -67,6 +67,8 @@ When neither flag is given, each subcommand picks its own default (typical: `7d`
 
 A single `--format` flag, not separate `--json` / `--csv` flags. `markdown` is the default because terminals are humans by default; agents pass `--format json` once.
 
+CLIs MAY implement a subset of these codecs. A CLI MUST implement `markdown` and `json`; `csv` is recommended but optional. The Status table tracks per-CLI implementation per codec, and `compat.Runner.SupportedFormats` lets the conformance bundle target the declared subset so a partial-codec exporter can adopt the bundle without an immediate `--format csv` failure. Declaring less than `{markdown, json}` is non-conforming — markdown is required for human terminal use, json is required for agent/script use, and either alone leaves one of the two use cases unserved.
+
 Output rules:
 
 - **stdout**: data only, in the requested format.