Remove max_cli_sessions — CLI session pool is now unbounded

The max_sessions cap in CLISessionManager was the only thing enforcing
a limit on concurrent CLI processes. Now that provider concurrency is
controlled at the streaming layer (PROVIDER_MAX_CONCURRENCY semaphore),
the CLI session pool cap is redundant and removed entirely.

Changes:
- cli/manager.py: remove max_sessions param, cap check, _cleanup_idle_sessions_unlocked, max_sessions from get_stats()
- config/settings.py: remove max_cli_sessions field
- api/app.py: remove max_sessions=settings.max_cli_sessions from CLISessionManager constructor
- messaging/handler.py: remove "Waiting for slot" status check; stats display no longer shows Max CLI
- .env.example: remove MAX_CLI_SESSIONS line
- tests/cli/test_cli.py: remove max_sessions args and assertion from manager tests
- tests/cli/test_cli_manager_edge_cases.py: remove two tests for cap/cleanup behavior
- tests/api/test_app_lifespan_and_errors.py: remove max_cli_sessions from all SimpleNamespace settings
- tests/config/test_config.py: remove max_cli_sessions isinstance assertion
- tests/conftest.py: remove max_sessions from mock stats
- tests/messaging/test_handler.py: merge slot/capacity tests into single new-conversation test; remove Max CLI assertion from stats test
- tests/messaging/test_handler_markdown_and_status_edges.py: remove "Waiting for slot" assertion; drop max_sessions from all stats mocks

https://claude.ai/code/session_014mrF1WMNgmNjtPBuoQHsbg

messaging/handler.py

@@ -668,13 +668,6 @@ class ClaudeMessageHandler:
             return self._format_status("🔄", "Continuing conversation...")
 
         # New conversation
-        stats = self.cli_manager.get_stats()
-        if stats["active_sessions"] >= stats["max_sessions"]:
-            return self._format_status(
-                "⏳",
-                "Waiting for slot...",
-                f"({stats['active_sessions']}/{stats['max_sessions']})",
-            )
         return self._format_status("⏳", "Launching new Claude CLI instance...")
 
     async def stop_all_tasks(self) -> int:
@@ -813,8 +806,6 @@ class ClaudeMessageHandler:
             + "\n"
             + ctx.escape_text(f"• Active CLI: {stats['active_sessions']}")
             + "\n"
-            + ctx.escape_text(f"• Max CLI: {stats['max_sessions']}")
-            + "\n"
             + ctx.escape_text(f"• Message Trees: {tree_count}"),
             fire_and_forget=False,
             message_thread_id=incoming.message_thread_id,