heph.nvim: rip out auto-spawn — connect-only plugin

The daemon is now an OS service (`heph daemon`); the plugin no longer spawns or
supervises one. Removes the managed-daemon machinery entirely.

- delete lua/heph/daemon.lua (spawn/ensure/stop_spawned/self-heal)
- init.lua: connect-only; probe `health` once and guide to `heph daemon start`
- rpc.lua: drop set_respawn + respawn-on-drop; a dropped connection just
  reconnects once (e.g. after `heph daemon restart`), never spawns
- config.lua: drop autostart/bin/db; stable socket fallback (data-dir, matches
  hephd::default_socket_path), keep $HEPH_SOCKET for dev isolation
- tests: spawn/wait_ready move into the e2e harness (test infra); rework
  managed_daemon_spec into a connect-only spec (connect / clean-fail / reconnect)

16 nvim e2e specs pass.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

heph.nvim/lua/heph/rpc.lua

@@ -182,13 +182,6 @@ function M.session()
   return M._default
 end
 
---- Register a hook that (re)ensures the daemon — called once to self-heal a
---- dropped connection before a single retry. `nil` disables self-heal (used when
---- autostart is off, so a connect-only setup fails loudly instead of respawning).
-function M.set_respawn(fn)
-  M._respawn = fn
-end
-
 local function is_connection_error(msg)
   msg = tostring(msg)
   return msg:find("connect", 1, true) ~= nil
@@ -196,16 +189,15 @@ local function is_connection_error(msg)
     or msg:find("timeout", 1, true) ~= nil
 end
 
---- Blocking call on the default session. If the call fails because the
---- connection is dead and a respawn hook is set, ensure the daemon and retry
---- once (the prior owner releases the DB lock on exit, so a respawn can claim it).
+--- Blocking call on the default session. The plugin is connect-only: on a
+--- dropped connection we drop the dead session and **reconnect once** (e.g. the
+--- daemon was restarted via `heph daemon restart`) — we never spawn a daemon.
 function M.call(method, params, opts)
   local ok, result = pcall(M.session().call, M.session(), method, params, opts)
   if ok then
     return result
   end
-  if M._respawn and is_connection_error(result) then
-    pcall(M._respawn)
+  if is_connection_error(result) then
     M.session():close() -- drop the dead connection so the retry reconnects
     return M.session():call(method, params, opts)
   end