fix(runtime): extend usage details to body and catalog discovery errors

[samzong]

Summary

Two gaps left after #171: a required request body with no body input failed with a bare invalid-command-usage message, and the catalog discovery commands hid their failures — commands show <unknown path> returned a generic command-failed error with a misleading local-configuration hint (exit 1), and an empty search query exited 0 with no output, indistinguishable from zero matches.

• resolveOperationBody required-body errors now carry a typed usage detail naming the accepted body inputs (--file, --set, --set-str, and body flags when present).
• commands show with an unknown path is a usage error (exit 2) whose detail points at the commands listing without echoing the requested path.
• A blank search query fails as a usage error stating the query must not be empty.

All details are static or spec-derived text under the existing error.detail contract (docs/contracts.md); no user input is echoed and no contract text changes.

Verification

• go test ./pkg/runtime/ -run TestBuild_RequiredBodyUsageErrorDetail and go test ./pkg/lathe/ -run 'TestCommandsShowUnknownPathUsageError|TestSearchEmptyQueryUsageError' — red before the fix, green after.
• make check and go test -race -count=1 ./... pass.
• Real-CLI probe: tokener built with a local replace shows Detail: request body required: pass --file, --set, --set-str, or a body flag for bare keys create, Detail: no generated command at that path; run \tokener commands` to list paths(exit 2) forcommands show keys nope, and Detail: search query must not be emptyforsearch ""`.

Compatibility

commands show with an unknown path changes from exit 1 (command failed) to exit 2 (usage), and a blank search query changes from exit 0 to exit 2. Error envelope shape, catalog schema, and the detail contract are unchanged.

Checklist

• Tests or focused verification cover the changed surface.
• User-facing behavior changes are documented.
• Generated output under internal/generated/, .cache/, and ad-hoc skills/<cli-name>/ directories is not committed.
• Commits are signed off when this is ready to merge.

[codspeed-hq]

Merging this PR will not alter performance

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✅ 21 untouched benchmarks

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_{Comparing fix/usage-detail-coverage (91a99e1) with main (4677bfa)}