feat: password Realm, single-chair login, shared parliament context (0.9.12)

Derive Realm keys from a user password (Argon2id, no private key on disk).
Login claims the sole chair with handover; peers share parliament context;
--new-project clears it. CLI peer/discover helpers and Docker alpine recipe
included; headless empty-answer retries raised for unattended runs.

Co-authored-by: Cursor <cursoragent@cursor.com>

include/cst_parliament.h

@@ -0,0 +1,51 @@
+#ifndef CST_PARLIAMENT_H
+#define CST_PARLIAMENT_H
+
+#include "cst_realm.h"
+
+#include <stddef.h>
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+/* Parliament is one shared whole across agents. Password unlock claims the
+ * single chair on the logging-in device; peers sync the same context. A new
+ * project replaces all parliament context. */
+
+typedef struct {
+  char parliament_id[64];
+  char project_id[64];
+  char chair_instance[64];
+  char chair_device[128];
+  unsigned long revision;
+  char status[32]; /* idle | open | dissolved */
+} cst_parliament_meta_t;
+
+int cst_parliament_path(char *out, size_t n);
+int cst_parliament_load(cst_parliament_meta_t *m);
+int cst_parliament_save(const cst_parliament_meta_t *m);
+
+/* Login: unlock Realm with password, enroll/ensure instance, claim chair,
+ * return handover summary (malloc'd) or NULL. */
+char *cst_parliament_login(const char *password, const char *instance_id,
+                           const char *device_label, char *err, size_t err_sz);
+
+/* Only one chair: previous chair is demoted; handover notes left in meta. */
+int cst_parliament_claim_chair(const char *instance_id, const char *device_label);
+
+/* Probe local environment as device-private context (malloc'd text). */
+char *cst_parliament_device_context(void);
+
+/* New project: wipe shared parliament context; keep Realm keys. */
+int cst_parliament_new_project(const char *project_id);
+
+/* Append/replace synchronized context blob (all agents share this file). */
+int cst_parliament_context_put(const char *text);
+char *cst_parliament_context_get(void);
+
+#ifdef __cplusplus
+}
+#endif
+
+#endif