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

cmd/vendorhash/main.go

@@ -12,7 +12,7 @@
 //	vendorhash check   exit non-zero if flakehashes.json is stale
 //	vendorhash update  recompute and rewrite flakehashes.json
 //
-// The JSON schema and goModFingerprint algorithm mirror upstream
+// The JSON schema and [goModFingerprint] algorithm mirror upstream
 // tailscale's tool/updateflakes so a future shared library extraction
 // is straightforward.
 package main
@@ -82,8 +82,8 @@ func usage() {
 	fmt.Fprintln(os.Stderr, "usage: vendorhash <check|update>")
 }
 
-// errStale signals to main that the check found a mismatch; it has
-// already printed a remediation message, so main should exit 1
+// errStale signals to [main] that the check found a mismatch; it has
+// already printed a remediation message, so [main] should exit 1
 // silently.
 var errStale = errors.New("vendor hash stale")