Split session management (9P and D-Bus) out of the monolithic system
prompts into dedicated files primed separately:
- session-9p.md: 9P filesystem session operations
- session-dbus.md: D-Bus method/signal session operations
System prompts are now server-agnostic. The install target copies
the appropriate session-*.md as session.md. Agent configs prime
"session" after "SYSTEM_PROMPT" to compose the full prompt.
This eliminates the N×2 variant matrix (one -9p and -dbus per model)
and makes adding new model prompts simpler.
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