crucible depot: Increment 1 core (status/push/verify/get/pull + local/cdn/bunny backends) (#30)

Implements Increment 1 of `docs/superpowers/specs/2026-07-04-crucible-depot-core-design.md`: a new stdlib-only `internal/depot` package wired into the dispatcher.

- `crucible depot status|push|verify|get|pull` with backends `local`/`cdn`/`bunny`; exit contract `0` clean / `1` drift / `2` unconfirmed-only / `64` usage / `70` internal.
- Presence is always probed against the real target (IPv4 1-byte range GET; HEAD is banned with a regression-tripwire test). `unconfirmed` is a distinct state, never collapsed into `missing`.
- No prompting anywhere: missing `BUNNY_STORAGE_*` env fails closed (read path included), enforced by a no-stdin test.
- Uploads: probe-then-PUT with `Checksum: <UPPER-sha>`; read/write key split (`BUNNY_STORAGE_READ_PASSWORD` falls back to `BUNNY_STORAGE_PASSWORD`).
- Field-driven fix included: per-probe transient retry (curl `--retry 2` equivalent) — without it a real 1490-blob CDN sweep reported 1222 false-unconfirmed; with it, 1490/1490 present in 74s, exit 0.
- Registry: depot `Wired: true`, joins the interactive menu; stale "(SeaweedFS)" wording removed.

Tests: unit + httptest fake-Bunny (probe sequence, Checksum header, key split, 428 throttling → exit 2) + local→bunny integration (drift → push → clean → idempotent no-second-PUT; incremental pull). `make check` green.

**Merge ordering:** this merges FIRST; the companion `sow-assets-manifest#crucible-depot-cutover` PR needs its flake input bumped to include this.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Reviewed-on: #30
Reviewed-by: xtul <mpiasecki720@protonmail.com>
Co-authored-by: vickydotbat <vickydotbat@tutamail.com>
Co-committed-by: vickydotbat <vickydotbat@tutamail.com>

internal/depot/manifest.go

@@ -0,0 +1,76 @@
+package depot
+
+import (
+	"fmt"
+	"os"
+	"path/filepath"
+	"regexp"
+
+	"gopkg.in/yaml.v3"
+)
+
+type manifestFile struct {
+	Assets []struct {
+		Path   string `yaml:"path"`
+		SHA256 string `yaml:"sha256"`
+		Size   int64  `yaml:"size"`
+	} `yaml:"assets"`
+}
+
+var sha256Pattern = regexp.MustCompile(`^[0-9a-f]{64}$`)
+
+func ValidSHA(s string) bool {
+	return sha256Pattern.MatchString(s)
+}
+
+func BlobKey(sha string) string {
+	return fmt.Sprintf("sha256/%s/%s/%s", sha[0:2], sha[2:4], sha)
+}
+
+func ReferencedSHAs(dir string) (map[string]int64, error) {
+	entries, err := os.ReadDir(dir)
+	if err != nil {
+		return nil, err
+	}
+
+	result := make(map[string]int64)
+	foundAny := false
+
+	for _, entry := range entries {
+		if entry.IsDir() {
+			continue
+		}
+		if filepath.Ext(entry.Name()) != ".yml" {
+			continue
+		}
+
+		foundAny = true
+		path := filepath.Join(dir, entry.Name())
+		data, err := os.ReadFile(path)
+		if err != nil {
+			return nil, err
+		}
+
+		var manifest manifestFile
+		if err := yaml.Unmarshal(data, &manifest); err != nil {
+			return nil, err
+		}
+
+		for _, asset := range manifest.Assets {
+			if !ValidSHA(asset.SHA256) {
+				return nil, fmt.Errorf("%s: asset %q: invalid sha256 %q", entry.Name(), asset.Path, asset.SHA256)
+			}
+
+			// Keep the largest size for each sha
+			if asset.Size > result[asset.SHA256] {
+				result[asset.SHA256] = asset.Size
+			}
+		}
+	}
+
+	if !foundAny {
+		return nil, fmt.Errorf("no *.yml files found in %s", dir)
+	}
+
+	return result, nil
+}