## Summary
- reuse workspace agent context within a single `runChat()` turn
- remove duplicate latest-build agent lookups between
`resolveInstructions()` and `getWorkspaceConn()`
- avoid the extra `GetWorkspaceAgentByID` fetch when the selected
`WorkspaceAgent` already has the needed metadata
- add focused internal tests for reuse and refresh-on-dial-failure
## Why
This came out of a 5000-chat / 10-turn scaletest on bravo against a
single workspace.
The run completed successfully, but coderd stayed DB-pool bound, and one
workspace-backed hot path stood out:
- `GetWorkspaceAgentsInLatestBuildByWorkspaceID ≈ 46.7k`
- `GetWorkspaceByID ≈ 48.0k`
- `GetWorkspaceAgentByID ≈ 2.2k`
Within one `runChat()` turn, chatd was rediscovering the same workspace
agent multiple times just to resolve instructions and open the workspace
connection.
## What this changes
This PR introduces a **turn-local** workspace context helper so a single
acquired turn can:
- resolve the selected workspace agent once
- reuse that agent for instruction resolution
- reuse the same `AgentConn` for workspace tools and reload/compaction
This stays turn-local only, so a later turn on another replica still
rebuilds fresh context from the DB.
## Expected impact
This is an incremental improvement, not a full fix.
It should reduce duplicated workspace-agent lookups and shave some DB
pressure from a hot path for workspace-backed chats, while preserving
multi-replica correctness.
## Testing
- `go test ./coderd/chatd/...`
- `golangci-lint run ./coderd/chatd/...`
Ethan
a33605df58