fix: pass agent context config explicitly instead of reading env (#24759)

The CODER_AGENT_EXP_* env vars are agent-internal options. When set
in the workspace environment they leak to MCP subprocesses and user
shells.

ReadEnvConfig() captures the values and ClearEnvVars() strips them
before the reinit loop, so config survives agent restarts. NewAPI
and ReadEnvConfig both use applyDefaults() to fill zero fields.
The chatd test passes config via agenttest.WithContextConfigFromEnv().

cli/agent.go

@@ -28,6 +28,7 @@ import (
 	"cdr.dev/slog/v3/sloggers/slogstackdriver"
 	"github.com/coder/coder/v2/agent"
 	"github.com/coder/coder/v2/agent/agentcontainers"
+	"github.com/coder/coder/v2/agent/agentcontextconfig"
 	"github.com/coder/coder/v2/agent/agentexec"
 	"github.com/coder/coder/v2/agent/agentssh"
 	"github.com/coder/coder/v2/agent/boundarylogproxy"
@@ -279,6 +280,11 @@ func workspaceAgent() *serpent.Command {
 			defer reinitCancel()
 			reinitEvents := agentsdk.WaitForReinitLoop(reinitCtx, logger, client)
 
+			// Read and strip env vars before the reinit
+			// loop so config survives agent restarts.
+			contextConfig := agentcontextconfig.ReadEnvConfig()
+			agentcontextconfig.ClearEnvVars()
+
 			var (
 				lastOwnerID uuid.UUID
 				lastErr     error
@@ -335,6 +341,7 @@ func workspaceAgent() *serpent.Command {
 					SocketPath:                 socketPath,
 					SocketServerEnabled:        socketServerEnabled,
 					BoundaryLogProxySocketPath: boundaryLogProxySocketPath,
+					ContextConfig:              contextConfig,
 				})
 
 				if debugAddress != "" {