all: apply godoc [Name] link conventions across comments

Every Go-identifier reference in // and /* */ comments now uses
godoc's [Name] linking syntax so pkg.go.dev and `go doc` render
them as clickable cross-references. No behaviour change.

Pattern applied across the tree:
  In-package         [Foo], [Foo.Bar]
  Cross-package      [pkg.Foo], [pkg.Foo.Bar]
  Stdlib             [netip.Prefix], [errors.Is], [context.Context]
  Tailscale          [tailcfg.MapResponse], [tailcfg.Node.CapMap],
                     [tailcfg.NodeAttrSuggestExitNode]

Skip rules:
  - File:line refs left as plain text
  - HuJSON wire keys inside backtick raw strings untouched
  - ACL/policy syntax tokens (tag:foo, autogroup:self, ...) not Go
    symbols, left as plain text
  - JSON/OIDC wire keys, gorm tags, RFC IPv6 placeholders, markdown
    link tags, decorative dividers — all left as-is

hscontrol/policy/v2/policy_test.go

@@ -735,10 +735,11 @@ func TestTagPropagationToPeerMap(t *testing.T) {
 	require.NoError(t, err)
 	require.NotEmpty(t, matchersForUser2, "MatchersForNode should return non-empty matchers (at least self-access rule)")
 
-	// Test ReduceNodes logic with the updated nodes and matchers
-	// This is what buildTailPeers does - it takes peers from ListPeers (which might include user1)
-	// and filters them using ReduceNodes with the updated matchers
-	// Inline the ReduceNodes logic to avoid import cycle
+	// Test [policy.ReduceNodes] logic with the updated nodes and matchers
+	// This is what [mapper.MapResponseBuilder.buildTailPeers] does - it takes peers from
+	// [state.State.ListPeers] (which might include user1) and filters them using
+	// [policy.ReduceNodes] with the updated matchers
+	// Inline the [policy.ReduceNodes] logic to avoid import cycle
 	user2View := user2Node.View()
 	user1UpdatedView := user1NodeUpdated.View()