Add the identity command and serve it on an opt-in HTTP info API

Build slackhq/nebula#1821 ("who is this vpn address?" over a local
HTTP+JSON socket) on the command registry, so the lookup is one command
on every surface and the socket is one more local API.

identity
- `identity [-address <vpn addr>[:port]]`: no address reports this node;
  with one, this node or the peer behind an established tunnel
- Certificate fields only: name, vpn addresses, networks, unsafe
  networks, groups, fingerprint, issuer, validity, cert version; a peer's
  fingerprint is the one nebula computed when it verified the cert
- Port accepted and stripped, 4in6 unmapped, so a RemoteAddr passes
  straight through
- No tunnel is a commands.NotFound; an empty -address is refused
- Lighthouse TXT lookup shares the owner and peer lookups, keeps answering
  with the initiating certificate, and no longer panics without a default cert
- Served on ssh and nebula ctl too

Info API
- Off until `local_api.info.enabled` with `socket` and optional quoted
  octal `socket_mode` (default "0600"); nothing derived
- Serves commands declaring Command.Identity plus help; others 403 naming
  their surfaces. TestInfoAPISurface pins the set to identity
- GET /v1 is the schema; GET /v1/<cmd>?<flag>=<value> runs one; body is
  the -json rendering via commands.Render; commands.PresentationFormat
  reads json/pretty from the query the way a line is read
- 400 commands.ParamError, 403 commands.WithheldError (what Resolve now
  returns for a withheld name, so help and dispatch type-check it too),
  404 NotFound or unknown, 500 Run error, refusal with no reason, or
  recovered panic with no detail; no redirects; every miss is JSON
- listen takes a mode: bind under umask 0077 then chmod; "0660" plus a
  searchable dir opens it to a group
- The chmod, at bind and on a mode-only reload, goes through chmodSocket,
  which never follows a link: a user who can write the socket's directory
  could otherwise swap the node for a link and have a root daemon put
  0660 or 0666 on any file. Only a socket we own is chmoded. On linux the
  node is held open O_PATH|O_NOFOLLOW, checked by fstat, and chmoded
  through /proc/self/fd, so the check and the chmod land on one inode;
  fchmodat2 is the fallback without /proc. On darwin and the BSDs it is
  an Lstat check and fchmodat(AT_SYMLINK_NOFOLLOW). clearStale Lstats
  too, so a dangling link is refused rather than followed by bind on
  darwin
- One endpoint type owns bind under mutex, bind-before-release move,
  context watch and stop/unlink; each server supplies what it serves
- Reloadable through the same reconcile shape as the control socket:
  mode applied in place, unparsable section keeps what runs, control
  socket path refused by name
- Wire returns Endpoints{Control, Info} with one Stop
- Windows unsupported for now (a pipe has no mode); warned once

Tests: identity lookups and shapes; API filter, status codes, panic
recovery, socket mode, rebind, cancellation; config parsing incl. bare
0660; Wire transitions; e2e of two nebulas with the info API resolving
the peer by address and port, through shared test.HTTPDo/HTTPGet helpers.
Goldens gain identity.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

commands/request.go

@@ -80,6 +80,31 @@ func presentationFormat(fl *flag.FlagSet) Format {
 	}
 }
 
+// PresentationFormat is presentationFormat for a surface that takes a command's options
+// by name rather than from a line: the info API's query string. Only the two presentation
+// flags are read from params; the command's own parameters are the caller's to pass to
+// Invoke, which refuses these two among them. A flag given with no value counts as set,
+// as -pretty does on a line, and one that is not a boolean is a ParamError.
+func PresentationFormat(params map[string]string) (Format, error) {
+	fl := flag.NewFlagSet("", flag.ContinueOnError)
+	addPresentationFlags(fl)
+
+	for _, k := range []string{FlagJSON, FlagPretty} {
+		v, ok := params[k]
+		if !ok {
+			continue
+		}
+		if v == "" {
+			v = "true"
+		}
+		if err := setFlag(fl, fl.Lookup(k), v); err != nil {
+			return FormatUnset, err
+		}
+	}
+
+	return presentationFormat(fl), nil
+}
+
 // flagIsTrue reports whether a bool flag was set to true. The flag package's bool value
 // is a flag.Getter whose Get returns the bool, so this never falls back to parsing text.
 func flagIsTrue(f *flag.Flag) bool {
@@ -108,10 +133,9 @@ func flagIsTrue(f *flag.Flag) bool {
 // parameters is refused rather than read as a preference.
 func Invoke(req Request) (Result, error) {
 	c := req.Command
-	names := slices.Sorted(maps.Keys(req.Params))
 
 	fl, fs, own := commandFlags(c)
-	if err := setParams(fl, string(c.Name), names, req.Params, own); err != nil {
+	if err := setParams(fl, string(c.Name), slices.Sorted(maps.Keys(req.Params)), req.Params, own); err != nil {
 		return Result{}, err
 	}