Enhancement: AgentTopology needs pruning and last_updated field

[mrveiss]

Problem

`agents/agent_orchestration/topology.py` (PR #2149) has no mechanism to limit topology growth:

• No `last_updated` timestamp on `AgentConnection` (design doc schema includes it)
• No pruning of stale/low-weight connections
• No max connection count or LRU eviction
• Growth is O(A² × T) where A = distinct agent IDs, T = task types

With free-form string agent IDs (not enum-constrained), connection count is unbounded. Compare with `mesh_pruner.py` which actively deletes edges below `min_weight`.

Discovered During

Code review of PR #2149 (AgentTopology)

Recommended Fix

1. Add `last_updated: Optional[datetime]` to `AgentConnection`
2. Add `prune_stale_connections(max_age_days, min_weight)` method
3. Document the bounded assumption (agent IDs map to ~13 AgentType values)

Impact

Severity: medium — bounded in current usage, unbounded if agent IDs become dynamic

[mrveiss]

Merged PR #2193. AgentTopology pruning + last_updated field.