v0.8.0: WebUI redesign, /web, workspace switch, 75% budget, no shell timeout

The front end is rebuilt on the design tokens the desktop app ships (surfaces,
color-mix borders, alpha hovers, accent): a 30-icon inline SVG sprite, dark and
light themes, layout switches as deep links (?side=0, ?panel=0, ?theme=light)
resolved before the first paint. The empty page is one line and leaves with the
first turn; the right rail keeps three cards - todo, file changes (+N -M) and
context (tokens, prompt cache hit rate). Both rails animate in and out, and a
collapsed rail widens its grid track to 0 instead of being removed, so the
transcript never slides into a zero-width column.

- src/webui.c: POST /api/workspace - re-root the agent at another directory
  (permission rules, metrics, AGENTS.md follow) and start a session there, as
  sessions live per directory; state also reports cached/cache_miss tokens
- src/agent_loop.c: cst_agent_set_workspace(), rebuilt prompt and re-measured
  budget; the auto-compact threshold is 75% of the window (was 60%)
- src/tui.c: /web forks a WebUI next to the TUI - separate process and a new
  session, because cst_webui_run() blocks and two writers corrupt one session
- src/os_compat.c, src/os_win.c: run_shell runs without a time limit unless
  CST_SHELL_TIMEOUT asks for one, so builds and long clones finish
- src/main.c, include/cst_webui.h: one CST_WEBUI_DEFAULT_PORT for --help, the
  docs and /web; Termux opens the browser through termux-open-url
- README.md, docs/webui.md, docs/env_openai.md: /web, /api/workspace, the new
  state fields, the 75% threshold and the layout deep links

tests/e2e_webui.sh green (permission round-trip, token paths, --web-dev);
make check green.

src/os_compat.c

@@ -519,7 +519,7 @@ int os_run_timeout_effective(void) {
   const char *e = getenv("CST_SHELL_TIMEOUT");
   if (e && atoi(e) > 0)
     return atoi(e);
-  return 120;
+  return 0; /* no limit: a build, a test suite or a long clone must finish */
 }
 
 double os_now_ms(void) {
@@ -662,7 +662,8 @@ int os_run_capture_ex(const char *cwd, const char *const argv[],
   int truncated = 0, killed = 0;
   int status = 0, reaped = 0, bg_killed = 0;
   double reaped_at = 0, killed_at = 0;
-  double deadline = os_now_ms() + os_run_timeout_effective() * 1000.0;
+  int timeout_s = os_run_timeout_effective();
+  double deadline = timeout_s > 0 ? os_now_ms() + timeout_s * 1000.0 : 0;
   for (;;) {
     struct pollfd pfd = {pipefd[0], POLLIN, 0};
     int pr = poll(&pfd, 1, 100);
@@ -692,7 +693,7 @@ int os_run_capture_ex(const char *cwd, const char *const argv[],
     }
     /* Checked on every pass, so a chatty command still stops promptly. */
     if (!reaped && !killed &&
-        ((g_run_cancel && *g_run_cancel) || now > deadline)) {
+        ((g_run_cancel && *g_run_cancel) || (deadline > 0 && now > deadline))) {
       killed = (g_run_cancel && *g_run_cancel) ? 2 : 1;
       killed_at = now;
       kill(-pid, SIGTERM);