feat(classification): data-sensitivity taxonomy + per-wing/repo tags (#50)

Implements the I1-prerequisite from #49/#50: the classification taxonomy
and the per-wing / per-repo tagging the capture server reads to derive a
target's sensitivity.

- Levels public < internal < confidential, ordered so "stricter wins"
  (spec §4.1 model C) is a plain max via Stricter().
- Tags read from an optional classification.yaml at the brain root
  (wings:/repos: maps). Absent file → defaults-only, not an error.
- Defaulting: client-* → confidential; hyperguild/homelab → internal;
  everything else → confidential. Fail-safe-to-strictest is the
  load-bearing property: a missing tag never silently downgrades.
- Config.Derive(Target) is the function the use-case calls; Wing/Repo
  are the per-kind helpers. ParseLevel rejects unknown tokens; a bad
  level in the config file is a hard load error.

Central classification.yaml (not _index.md frontmatter, not gitea repo
topics): classifying a repo needs no live Gitea client, so #50 has no
dependency on the tracker work (#52); it's auditable in one place; and
it avoids BuildWingIndex clobbering a wing's regenerated _index.md.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

specs/capture-bdd-spec.md

@@ -203,6 +203,12 @@ binding design decisions for the build.
    - **Prerequisite (new build work):** a classification taxonomy (e.g. `public` /
      `internal` / `confidential`) and a per-wing / per-repo classification tag the server can read.
      This must exist before the I1 gate is load-bearing. Tracked as a sub-task of #49.
+     - **Implemented (#50):** taxonomy `public < internal < confidential` (ordered so "stricter wins"
+       is `max`) in `ingestion/internal/classification/`. Tags are read from an optional
+       `classification.yaml` at the brain root (`wings:` / `repos:` maps); absent entries fall to
+       built-in defaults (`client-*` → confidential; `hyperguild`/`homelab` → internal; everything
+       else → **confidential, fail-safe**). `Config.Derive(Target)` is the function the use-case
+       calls. See brain `wiki/hyperguild/decisions/capture-classification-taxonomy`.
    - Rationale: composes with decision 2; fails safe; honours a caller flagging something *more*
      sensitive than its destination. Pure caller-trust (A) was rejected — it makes the gate theatre.