Add a read-only/no-tools mode, cycled via command mode

[shellicar]

Summary

• A new command-mode key (o) cycles a session mode: normal, read-only, no-tools.
• Read-only offers only tools with no write/delete/escalate effect; no-tools offers none. A narrowed tool is absent from the model's tool list, not denied after being offered.
• The model is told when its available tools change, as a persisted reminder distinct from the read/delete/escalate/write scope.
• The status line shows the active mode when it isn't normal.
• Two tools (Ref, and the history search/read tools) are reclassified as having no filesystem effect, so read-only mode still allows them.