docs(specs): add dot_alfred sidecar design doc

First entry in the new specs/ directory. Specifies the layout and
semantics of the per-show .alfred/ sidecar that will back the future
concrete TVShowRepository:

- One .alfred/ directory per show, containing show.yaml + one
  season_NN.yaml per season (zero-padded, season_00 for Specials).
- Per-episode entries store file size + mtime so cache lookups skip
  a full ffprobe rescan when nothing changed.
- Self-healing on drift (file missing/modified/new) without raising.
- Atomic writes via temp file + os.replace().
- Phased implementation plan (builder + freeze first, then
  serializer, then cache validator, then repo, then wiring).

No code yet — spec only, awaiting review before the implementation
phases. Companion entry in CHANGELOG (Added).

CHANGELOG.md

@@ -15,6 +15,19 @@ callers).
 
 ## [Unreleased]
 
+### Added
+
+- **Design doc for `.alfred/` sidecar persistence
+  (`specs/dot_alfred.md`).** First entry in the new `specs/` directory.
+  Specifies a per-show `.alfred/` directory holding a `show.yaml` and
+  one `season_NN.yaml` per season, used by the upcoming concrete
+  `TVShowRepository` to cache parse/probe results and avoid full
+  rescans on every library read. Covers schema, naming conventions,
+  cache invalidation strategy (size + mtime), self-healing on
+  drift, atomicity (`os.replace`), edge cases (legacy folders,
+  corrupted sidecars, manual file removal), and a phased
+  implementation plan. No code yet — spec only.
+
 ### Internal
 
 - **`specs/` is now tracked.** The repo-level `.gitignore` had a