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/handlers_test.go

@@ -15,8 +15,8 @@ import (
 var errTestUnexpected = errors.New("unexpected failure")
 
 // TestHandleVerifyRequest_OversizedBodyRejected verifies that the
-// /verify handler refuses POST bodies larger than verifyBodyLimit.
-// The MaxBytesReader is applied in VerifyHandler, so we simulate
+// /verify handler refuses POST bodies larger than [verifyBodyLimit].
+// The [http.MaxBytesReader] is applied in [Headscale.VerifyHandler], so we simulate
 // the same wrapping here.
 func TestHandleVerifyRequest_OversizedBodyRejected(t *testing.T) {
 	t.Parallel()
@@ -47,7 +47,7 @@ func TestHandleVerifyRequest_OversizedBodyRejected(t *testing.T) {
 		"oversized body must surface 413")
 }
 
-// errorAsHTTPError is a small local helper that unwraps an HTTPError
+// errorAsHTTPError is a small local helper that unwraps an [HTTPError]
 // from an error chain.
 func errorAsHTTPError(err error) (HTTPError, bool) {
 	var h HTTPError