fix: set AGENT_HOME env var for agent processes

The $AGENT_HOME environment variable was referenced by skills (e.g.
para-memory-files) but never actually set, causing runtime errors like
"/HEARTBEAT.md: No such file or directory" when agents tried to resolve
paths relative to their home directory.

Add agentHome to the paperclipWorkspace context in the heartbeat service
and propagate it as the AGENT_HOME env var in all local adapters.

Co-Authored-By: Paperclip <noreply@paperclip.ing>

server/src/services/heartbeat.ts

@@ -1523,6 +1523,7 @@ export function heartbeatService(db: Db) {
       repoRef: executionWorkspace.repoRef,
       branchName: executionWorkspace.branchName,
       worktreePath: executionWorkspace.worktreePath,
+      agentHome: resolveDefaultAgentWorkspaceDir(agent.id),
     };
     context.paperclipWorkspaces = resolvedWorkspace.workspaceHints;
     const runtimeServiceIntents = (() => {