fix(skillpack): address review — symlink read, git arg-injection, dup-subscribe, nil panics

Real issues from the PR review:
- security: readTree now skips symlinks (a pack with SKILL.md -> /etc/passwd
  or scripts/x -> ../../.ssh/id_rsa could read host files); covers file and
  dir symlinks, incl. within a git subpath
- security: GitSource rejects url/ref beginning with '-' (git arg injection)
  and clones with '--' separator; --filter=blob:none (blobless partial clone)
  instead of full-history clone
- correctness: Subscribe no longer swallows a non-ErrNotFound store error from
  GetByName (would create a duplicate subscription); handles *GitSource as well
  as GitSource in the URL/subpath extraction
- correctness: pinTo no longer renames a subscription, so Apply can't silently
  collide two subscriptions when an upstream pack changes its name
- validation: isKebab rejects leading/trailing/consecutive hyphens; BOM-
  prefixed SKILL.md now parses (matches the doc comment)
- robustness: Catalog/Activate/renderPackBody/Stage guard nil/malformed packs
- test cleanup: Syncer.Store field renamed Cache (collided with the Store
  interface); test NewID returns distinct ids
- tests: symlink-skip, BOM, strict-kebab, nil-pack-safety

Deferred (advisory perf, documented): PackCache stores raw trees so activation
re-parses; CheckAll is serial. Both fine at expected scale.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

skillpack/sync.go

@@ -2,6 +2,7 @@ package skillpack
 
 import (
 	"context"
+	"errors"
 	"fmt"
 	"time"
 
@@ -14,7 +15,7 @@ import (
 // — the only call that changes the bytes an agent runs is Apply, always with an
 // actor recorded.
 type Syncer struct {
-	Store PackCache // content store for pinned trees
+	Cache PackCache // content store for pinned trees
 	Subs  Store     // subscription metadata store
 
 	// SourceFor builds the Source for a stored subscription. A host overrides
@@ -73,7 +74,7 @@ func (y *Syncer) fetchPack(ctx context.Context, src Source, ref string) (*Pack,
 	if err != nil {
 		return nil, "", err
 	}
-	if err := y.Store.Put(ctx, pack.Digest, pack.Tree); err != nil {
+	if err := y.Cache.Put(ctx, pack.Digest, pack.Tree); err != nil {
 		return nil, "", err
 	}
 	return pack, sourceRef, nil
@@ -87,21 +88,32 @@ func (y *Syncer) Subscribe(ctx context.Context, src Source, trackRef, by string)
 	if err != nil {
 		return nil, err
 	}
-	if existing, err := y.Subs.GetByName(ctx, pack.Manifest.Name); err == nil {
+	existing, err := y.Subs.GetByName(ctx, pack.Manifest.Name)
+	if err == nil {
 		return nil, fmt.Errorf("skillpack: already subscribed to %q (id %s)", pack.Manifest.Name, existing.ID)
 	}
+	if !errors.Is(err, ErrNotFound) {
+		// A transient store error must NOT fall through to creating a row — that
+		// would produce a duplicate subscription the uniqueness check missed.
+		return nil, fmt.Errorf("skillpack: checking for existing subscription %q: %w", pack.Manifest.Name, err)
+	}
 
 	sub := &Subscription{
 		ID:         y.newID(),
+		Name:       pack.Manifest.Name,
 		SourceKind: src.Kind(),
 		SourceURL:  src.String(),
 		TrackRef:   trackRef,
 		Enabled:    true,
 	}
-	// Store the raw URL/subpath (String() may combine them for display).
-	if gs, ok := src.(GitSource); ok {
-		sub.SourceURL = gs.URL
-		sub.Subpath = gs.Subpath
+	// Store the raw URL + subpath separately (String() may combine them for
+	// display). GitSource methods have value receivers, so a caller may pass
+	// either GitSource or *GitSource — handle both.
+	switch gs := src.(type) {
+	case GitSource:
+		sub.SourceURL, sub.Subpath = gs.URL, gs.Subpath
+	case *GitSource:
+		sub.SourceURL, sub.Subpath = gs.URL, gs.Subpath
 	}
 	sub.pinTo(pack, sourceRef, by, y.now())
 	if err := y.Subs.Save(ctx, sub); err != nil {
@@ -175,7 +187,7 @@ func (y *Syncer) Apply(ctx context.Context, id, by string) (*Subscription, error
 	if !sub.HasPending() {
 		return nil, fmt.Errorf("skillpack: %q has no pending update to apply", sub.Name)
 	}
-	tree, err := y.Store.Get(ctx, sub.PendingDigest)
+	tree, err := y.Cache.Get(ctx, sub.PendingDigest)
 	if err != nil {
 		return nil, fmt.Errorf("skillpack: pending tree for %q missing from cache: %w", sub.Name, err)
 	}