Phase 1 of the Scalable Approach for Adding New IDE Session Types RFC,
tracked in #27410.
Gives agent session accounting one canonical vocabulary for app names, so
later work can report a count per app without the agent and the server
disagreeing about what an app is called.
- Adds codersdk/appname.go: NormalizeAppName for storage and lookup, and
AppFamilyName for the places that need a bounded value, such as a
Prometheus label or the Connection_Type enum.
- agentssh.Server keeps session counters in a map keyed by normalized name
instead of three fixed atomics, dropping entries at zero.
- Drops MagicSessionType. Sessions carry plain app names, and unrecognized
ones flow through as themselves rather than collapsing to unknown.
Wire behavior is unchanged: the agent still reports the four session_count_*
fields over Agent API 2.10.
