refactor: normalize workspace agent session metadata - #28124

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 coderd/idemetadata, a leaf package importable from both agent and coderd code, holding the canonical names and their normalization: null bytes stripped, truncated to 64 runes, trimmed, lowercased, hyphens folded to underscores.
• Adds a family grouping for the places that need a bounded value, such as a Prometheus label or the Connection_Type enum. It is never used for storage, so a missing alias only costs an unknown label.
• agentssh.Server keeps its session counters in a map keyed by normalized name instead of three fixed atomics, dropping entries at zero so idle apps are not reported.
• extractMagicSessionType canonicalizes rather than classifies, so an unrecognized type flows through as itself.
• JetBrains channel tracking takes a start/end callback instead of a counter pointer.

Wire behavior is unchanged: the agent still reports the four session_count_* fields over Agent API 2.10, Collect translates only the four canonical names those fields exist for, and TestAgent_Stats_SSH, TestAgent_Stats_Magic, and TestAgent_Stats_ReconnectingPTY still assert the old fields untouched.

Stack

1. this PR: normalize agent session metadata
2. feat: add workspace agent session count protocol #28125: add the Stats.session_counts protocol field
3. perf: store workspace agent session counts as JSONB #28126: store session counts as JSONB (the only migration in the stack)
4. feat: report arbitrary workspace agent session types #28127: report arbitrary session types