Each YAML file at `.mdsmith/kinds/<name>.{yaml,yml}` now declares
one kind. The basename is the kind's name; the body is the full
`KindBody` (schema, rules, path-pattern, extends). Inline
`kinds.<name>:` in `.mdsmith.yml` stays a first-class source —
projects can mix file-defined and inline kinds freely. The same
name in both is a config error naming both sources so a merged
kind cannot defeat the "read one file to know one kind" property
this surface ships.

`discoverKinds` walks the directory at load time and rejects bad
basenames (`[a-z][a-z0-9-]*`), subdirectories, `.yaml`/`.yml`
extension collisions, and unknown top-level keys (strict
decoding). The merged kind layer threads through `ValidateKinds`
unchanged so the pairwise schema-source mutex and the
`extends:` cycle detector apply uniformly across sources.

Every kind body now carries a `SourcePath` populated by `Load`
(the `.mdsmith.yml` path for inline kinds, the kind-file path
for file kinds). Provenance threads that path through
`LayerEntry.SourcePath` and `ResolvedKind.SourcePath`, and
`mdsmith kinds resolve` / `kinds show` print a `defined-in` line
next to each kind they report. The JSON shape carries
`source-path` on both the body and per-resolved-kind entries.

The repository's own `.mdsmith.yml` migration is deferred:
the pinned `mdsmith` binary this repo lints itself with does
not yet support `.mdsmith/kinds/`. Migration is scheduled for
after the next release bumps the pinned version. Plan 209 is
filed as the follow-up for the parallel
`.mdsmith/conventions/<name>.yaml` slot.

https://claude.ai/code/session_01Ctz5CK1Zhz99xn4RYtmDGn

codecov/patch reported `∅` for the Go component on PR #391 because
the new helpers had uncovered defensive branches that pulled patch
coverage below the project baseline. Close them by either driving
the branch with a test or removing it:

- `mergeKindFiles`: drop the `cfgPath == ""` early return.
  Load is the only caller and always passes a non-empty path.
  CLAUDE.md's "defensive only when you can drive it red/green"
  rule applies here — no test could realistically reach it.
- `discoverKinds`: add tests for the non-IsNotExist ReadDir
  branch (plant a regular file at `.mdsmith/kinds`), the
  non-YAML-extension skip branch (drop a `.md` alongside),
  the YAML anchor/alias rejection branch, and the
  malformed-YAML decode branch. Each test names the offending
  file in the assertion so future refactors keep the
  file-naming contract.
- `parseKindFile`: add a chmod-0000 read-error test. Skips on
  test users (root) where mode-0000 files remain readable;
  CI runs as `runner`, where the branch is exercised.
- `Load`: add a propagation test so a discovery error from a
  bad basename surfaces as the `loading kind files` wrap on
  the Load surface.
- `WriteBodyText` (kindsout): add a failingWriter test for the
  new `defined-in:` line so its error path matches the rest
  of the writer surface.

After these, the three new helpers (`discoverKinds`,
`mergeKindFiles`, `parseKindFile`) hit 100% / 100% / ~91%
(the last branch covered in CI). `WriteBodyText` is back at
100%.

https://claude.ai/code/session_01Ctz5CK1Zhz99xn4RYtmDGn