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/types/testlog.go

@@ -17,18 +17,18 @@ const EnvTestLogLevel = "HEADSCALE_TEST_LOG_LEVEL"
 // emits zerolog output, so this init() runs once per test binary and is
 // the only place that needs to know about test logging configuration.
 //
-// Default: ErrorLevel (silent in green-path runs, real errors still surface).
+// Default: [zerolog.ErrorLevel] (silent in green-path runs, real errors still surface).
 // Override: HEADSCALE_TEST_LOG_LEVEL=debug (or trace, info, warn, disabled).
 //
-// Production binaries are unaffected because testing.Testing() returns false
-// outside of test execution. The same testing.Testing() pattern is already
+// Production binaries are unaffected because [testing.Testing] returns false
+// outside of test execution. The same [testing.Testing] pattern is already
 // used in hscontrol/db/users.go and hscontrol/db/node.go, so importing the
 // testing package here is consistent with existing project conventions.
 //
 // Pitfalls:
-//   - log.Fatal still calls os.Exit and log.Panic still panics regardless of
+//   - log.Fatal still calls [os.Exit] and log.Panic still panics regardless of
 //     level — only the rendered message is suppressed.
-//   - Local buffer loggers (zerolog.New(&buf)) are also gated by the global
+//   - Local buffer loggers ([zerolog.New] with a buffer) are also gated by the global
 //     level. Tests that assert on log output (currently only
 //     hscontrol/util/zlog) re-enable trace level via their own init_test.go.
 func init() {