fix(core): resolve workspace for cron prompt-mode jobs in multi-workspace mode

[Cigarrr]

Summary

In multi-workspace mode, ExecuteCronJob() for prompt (non-shell) jobs always used the global e.agent and passed an empty workspaceDir to processInteractiveMessageWith(). This caused the Claude Code agent to start with the wrong working directory (typically ~/.cc-connect) instead of the workspace bound to the cron job's channel.

The fix mirrors handleMessage()'s workspace resolution logic:

1. Extract channelID from the cron job's sessionKey
2. Resolve the workspace binding via resolveWorkspace()
3. Get a workspace-specific agent/sessions via workspaceContext()
4. Pass the resolved agent and workspaceDir to processInteractiveMessageWith()

job.WorkDir, when explicitly set, takes highest priority as an override.

Priority chain: job.WorkDir (explicit) → workspace binding resolution → global agent fallback

Closes #555

Related

• fix(core): strip workspace prefix from cron/heartbeat session keys #474 — fixed workspace-prefixed session key parsing in ExecuteCronJob(), but did not add workspace agent resolution
• fix: persist multi-workspace /dir overrides without cross-channel leakage #505 — introduced resolveChannelWorkDir() and workspaceContext() which provide the infrastructure this fix wires into

Test plan

• go test ./core/ -count=1 — all tests pass
• go build ./... — clean build
• Manually verified: cron prompt jobs now execute in the correct workspace directory (confirmed via gitStatus in agent context)
• Verified job.WorkDir override takes precedence when explicitly set
• Verified non-multi-workspace mode is unaffected (falls through to global agent)

🤖 Generated with Claude Code

[chenhg5]

LGTM. Cron job now correctly resolves workspace context in multi-workspace mode — respects job.WorkDir priority over channel binding. Proper session routing.

PR: #556