add mermaid flowchart of agent boot

checkbox-ng/plainbox/impl/session/agent_bootup.md

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+# Checkbox Agent bootup process
+
+
+```mermaid
+graph TD;
+  proc("agent process starts")
+  load("load previous session")
+  resume("resume the previous session")
+  resume_crashed("resume the previous session")
+  auto_session{"was the previous session non-interactive"}
+  mark_pass("mark last running job as passing")
+  mark_crash("mark last running job as crashing")
+  idle("go into idle state")
+  listen("listen for the controllers")
+  proc --> load
+  last_job{"was the last job a `noreturn` job?"}
+  load -->last_job
+  last_job-->|yes| resume
+  resume --> mark_pass
+  
+  last_job-->|no| auto_session
+  auto_session-->|no| idle
+  idle --> listen
+  mark_pass --> listen
+
+  auto_session-->|yes| resume_crashed
+  resume_crashed --> mark_crash
+  mark_crash --> listen
+
+  
+
+
+```
+