feat: capture SSH exec commands (PLAN.md 4.4)

Bots often send commands via `ssh user@host <command>` (exec request)
rather than requesting an interactive shell. These were previously
rejected silently. Now exec commands are captured, stored on the session
record, and displayed in the web UI session detail page.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

internal/storage/migrations/004_add_exec_command.sql

@@ -0,0 +1 @@
+ALTER TABLE sessions ADD COLUMN exec_command TEXT;