config: normalize ssh_key_path — expand ~/, reject non-absolute

Bug: resolveSSHKeyPath returned a configured ssh_key_path verbatim.
That meant:

- ssh_key_path = "~/.ssh/id_ed25519" kept the literal "~" — downstream
  readers (internal/guest/ssh.go, internal/daemon/image_seed.go,
  internal/daemon/vm_authsync.go, internal/cli/ssh.go) do raw
  os.ReadFile on the path and fail at runtime with a path that looks
  fine but isn't.
- ssh_key_path = "id_ed25519" (relative) silently worked or didn't
  depending on the daemon's cwd — the daemon process's cwd is not
  the user's shell cwd, so behavior was non-obvious.

Fix: add normalizeSSHKeyPath() run over configured values. It:

  - expands "~/..." against $HOME
  - rejects bare "~" (ambiguous)
  - rejects "~user/..." (we don't do user-tilde)
  - rejects relative paths outright
  - returns filepath.Clean'd absolute paths

Tests cover the accepting case (home-anchored expansion) and every
rejection branch via a table-driven subtests.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

internal/config/config.go

@@ -258,7 +258,7 @@ func validateFileSyncMode(mode string) error {
 func resolveSSHKeyPath(layout paths.Layout, configured string) (string, error) {
 	configured = strings.TrimSpace(configured)
 	if configured != "" {
-		return configured, nil
+		return normalizeSSHKeyPath(configured)
 	}
 	// Key lives under the state dir, not the config dir. The daemon's
 	// ensureVMSSHClientConfig scrubs ConfigDir/ssh on every Open as
@@ -275,6 +275,44 @@ func resolveSSHKeyPath(layout paths.Layout, configured string) (string, error) {
 	return ensureDefaultSSHKey(filepath.Join(sshDir, "id_ed25519"))
 }
 
+// normalizeSSHKeyPath validates and canonicalises a user-configured
+// ssh_key_path. Accepts:
+//
+//   - absolute paths ("/home/me/keys/id_ed25519")
+//   - home-anchored paths ("~/keys/id_ed25519") — expanded against $HOME
+//
+// Rejects:
+//
+//   - bare "~" (ambiguous — expand to what?)
+//   - "~other/foo" (we only expand the current user's home)
+//   - relative paths ("id_ed25519", "./keys/id_ed25519") — these are
+//     ambiguous because the daemon's cwd isn't the user's shell cwd,
+//     and readers in internal/guest + internal/cli do raw os.ReadFile
+//     on the path without re-resolving against a known anchor
+func normalizeSSHKeyPath(raw string) (string, error) {
+	raw = strings.TrimSpace(raw)
+	if raw == "" {
+		return "", nil
+	}
+	if raw == "~" {
+		return "", fmt.Errorf("ssh_key_path %q: bare '~' is not supported, point at a specific key file", raw)
+	}
+	if strings.HasPrefix(raw, "~") && !strings.HasPrefix(raw, "~/") {
+		return "", fmt.Errorf("ssh_key_path %q: only '~/' is expanded, not '~user/'", raw)
+	}
+	if strings.HasPrefix(raw, "~/") {
+		home, err := os.UserHomeDir()
+		if err != nil {
+			return "", fmt.Errorf("ssh_key_path %q: expand ~/: %w", raw, err)
+		}
+		raw = filepath.Join(home, strings.TrimPrefix(raw, "~/"))
+	}
+	if !filepath.IsAbs(raw) {
+		return "", fmt.Errorf("ssh_key_path %q: must be absolute (start with '/') or home-anchored (start with '~/')", raw)
+	}
+	return filepath.Clean(raw), nil
+}
+
 func ensureDefaultSSHKey(path string) (string, error) {
 	if err := os.MkdirAll(filepath.Dir(path), 0o700); err != nil {
 		return "", err