Usability pass: tool registry, TUI polish, saved keys and settings

Tools
- Table-driven tool registry with permission classes and default/minimal
  profiles; new tools: edit_file, list_dir, glob, grep, delete_file,
  move_file, todo_write, web_fetch, ask_user (plus read_file paging).
- Built-in Myers diff; write tools return unified diffs.
- Persistent permission rules (.agent/permissions.json), per-turn undo
  snapshots (.agent/undo), AGENTS.md project notes, auto-compact at 80%
  of the model context, cancellable run_shell with process-group kill.

TUI
- Table-driven slash commands: /provider /key /effort /status /tools
  /permissions /yolo /undo /retry /diff /export /copy /rename /delete
  /init /max-steps /todo, with / and @ completion.
- Permission modal with diff and prefix rules, ask_user modal, todo
  panel, header with model/effort/context, bracketed paste, line editing,
  wide-char cursor, NO_COLOR and ASCII fallback; Esc never quits.

Startup and configuration
- API keys are entered in the TUI (hidden) and saved per provider in
  <config>/credentials (0600); env variables are only a fallback.
- Last provider, per-provider model and reasoning effort are remembered
  in <config>/settings.json; --effort/-e and /effort send
  reasoning_effort (low|medium|high|max) to models that accept it.
- DeepSeek preset uses deepseek-flash / deepseek-v4-pro.
- Session flags --new/--continue/--resume/--list-sessions, --yolo,
  --confirm.

Compatibility and tests
- ncurses detection, macOS/Homebrew support, make install, XDG config
  dir, POSIX cleanups; Gitea CI (gcc/clang/asan/musl/macOS).
- Unit tests for diff, fs tools, registry, undo/perms and config;
  e2e fixtures multi_file_refactor and ask_or_todo; vendored cJSON.

include/os_compat.h

@@ -24,6 +24,7 @@ int os_path_under(const char *root, const char *path);
 
 /* --- dirs / config --- */
 int os_mkdir(const char *path, int mode);
+int os_mkdir_p(const char *path, int mode);
 int os_config_dir(char *out, size_t out_sz);
 int os_temp_dir(char *out, size_t out_sz);
 /* Create unique temp file; writes path into out; returns fd or -1. Caller closes. */
@@ -56,6 +57,15 @@ int os_run_capture(const char *cwd, const char *const argv[], size_t max_out,
 int os_run_shell_c(const char *cwd, const char *cmd, size_t max_out,
                    os_run_result_t *r);
 
+/* Child processes get stdin=/dev/null, their own process group, and are
+ * killed after the timeout (CST_SHELL_TIMEOUT seconds, default 120) or when
+ * *flag becomes non-zero. Exit code 124 = timeout, 130 = cancelled. */
+void os_set_cancel_flag(volatile int *flag);
+void os_set_run_timeout(int seconds);
+
+/* Monotonic milliseconds (arbitrary epoch). */
+double os_now_ms(void);
+
 /* --- misc --- */
 void os_localtime(const time_t *t, struct tm *out);
 int os_getpid(void);