CLI quiet-mode and directory-summary consistency gaps: `build --quiet` prints summary while lint/check/fmt suppress; `lint` has no directory summary

[dean0x]

Background

Two pre-existing consistency gaps in CLI quiet/summary behavior were noted during the PR #196 review. Neither was introduced by PR #196, but both were identified in files within the diff and are now tracked here.

Ledger IDs: #46 (consistency-cli/build.rs:1491-quiet_summary_unguarded), #48 (consistency-cli/lint.rs:849-no_directory_summary)
Files: mds-cli/src/build.rs:1491, mds-cli/src/lint.rs:849-937
Originating reviewer: Consistency

Issue A — build --quiet prints a summary line (#46)

mds-cli/src/build.rs:1491 emits a compilation summary (N compiled, M failed) unconditionally — it does not check the --quiet flag. mds lint --quiet, mds fmt --quiet, and mds check --quiet all suppress their equivalent summary lines. A developer using build in a quiet CI pipeline still gets summary noise.

This is a deliberate divergence currently, not an oversight — but no code comment documents it, and it was not called out in the CHANGELOG as intentional behavior.

Issue B — lint has no directory summary (#48)

mds lint dir/ produces per-file diagnostics but no trailing summary line (e.g., "N files checked, M with errors, K warnings"). Every other subcommand in directory mode (build, fmt, check) prints a summary. The omission means a quiet mds lint dir/ exit 0 gives no feedback at all about how many files were processed.

Adding a summary is a new output contract — needs a decision about format and whether it is quiet-suppressed — so it is a feature addition, not a fix.

Why deferred from PR #196

Both are pre-existing gaps not introduced by PR #196. #46 is a behavior change to --quiet semantics for build; #48 is a new output line. The right fix is to settle them together in a coordinated "quiet and summary audit" PR after the v0.4.0 release.

Proposed approach

1. Decide whether build --quiet should suppress its summary (likely yes — align with siblings).
2. Design the lint directory summary format: "N files checked: M error, K warning" + exit code spec.
3. Update README.md and --help text for both subcommands together.
4. File CHANGELOG entry under Changed (behavior change for build --quiet).

Acceptance Criteria

• mds build --quiet dir/ produces no output on success (currently: prints summary line)
• mds lint dir/ prints a directory summary on completion (currently: none)
• mds lint dir/ --quiet suppresses the summary line (consistent with build/check/fmt)
• --help text updated to document --quiet behavior consistently
• README.md updated to reflect the final quiet behavior for all subcommands

Deferred from: PR #196
Bundle: Pair with #42 (quiet-flag help-text family fix) which addresses the text inconsistencies.