refactor(cli): split agent/cli.py into agent/cli/ package

Restructures the 3216-line agent/cli.py into a focused package
while preserving every existing surface verbatim.

Legacy compatibility:
- _legacy.py shim keeps every subcommand (serve, run, hypothesis,
  alpha, memory, provider, swarm, sessions, ...) reachable through
  the same CLI flags and console-script entry.
- Public symbols (cmd_*, _INIT_ENV_PATH, Confirm, AGENT_DIR, ...)
  re-exported through cli.<name>; `from cli import *` exposes 76
  symbols (was 1).
- Package-level cli.RUNS_DIR / cli.SESSIONS_DIR monkeypatches now
  propagate into _legacy before any cmd_* runs.

New scaffolding:
- agent/cli/main.py interactive REPL front door
- agent/cli/commands/ slash router + handlers (15 commands)
- agent/cli/components/ reusable Rich UI building blocks
- agent/cli/{theme,intro,onboard,input,completer,stream}.py
- agent/cli/__main__.py restores `python -m cli`
- agent/cli/_version.py reads importlib.metadata.version() so
  --version output and the banner stay aligned with pyproject.toml

Production wiring:
- New FastAPI middleware in api_server.py serves
  frontend/dist/index.html on browser navigation to /runs/{id}
  and /correlation, regex-narrowed so /runs/{id}/code and
  /runs/{id}/pine API routes stay reachable.
- frontend/vite.config.ts dev proxy mirrors the same narrowing.
- scripts/dev backend starts via package-safe
  `python -c 'import cli; cli.main(...)'`.

Chat-gate narrowed: chat --help / chat -h / chat --max-iter <bad> /
chat extra correctly reach legacy argparse instead of being
swallowed by the new REPL.

Tests: 2271 passed, 43 skipped (focused CLI/SPA test files:
test_cli_interactive.py 67 + test_spa_deep_link.py 13 + existing
CLI memory/init/hypothesis 48 = 128).

scripts/dev

@@ -130,7 +130,7 @@ wait_for_url() {
 }
 
 cmd_up() {
-  start_service backend env PYTHONPATH="$ROOT/agent" "$PYTHON_BIN" "$ROOT/agent/cli.py" serve \
+  start_service backend env PYTHONPATH="$ROOT/agent" "$PYTHON_BIN" -c 'import cli, sys; raise SystemExit(cli.main(sys.argv[1:]))' serve \
     --host "$BACKEND_HOST" \
     --port "$BACKEND_PORT"
   start_service frontend bash -lc '
@@ -159,7 +159,7 @@ cmd_restart() {
   case "$service" in
     backend)
       stop_service backend
-      start_service backend env PYTHONPATH="$ROOT/agent" "$PYTHON_BIN" "$ROOT/agent/cli.py" serve \
+      start_service backend env PYTHONPATH="$ROOT/agent" "$PYTHON_BIN" -c 'import cli, sys; raise SystemExit(cli.main(sys.argv[1:]))' serve \
         --host "$BACKEND_HOST" \
         --port "$BACKEND_PORT"
       wait_for_url backend "http://127.0.0.1:$BACKEND_PORT/health" 30