fix(flow-next): check depends_on_epics in cmd_ready

[possibilities]

Hello, really enjoying this project, thanks for making it and making it public (also huge compliments on your website!). If you're interested I made this patch for an issue I ran into.

I use flowctl's CLI commands directly rather than through Ralph/flow-next-work. The ready command doesn't check depends_on_epics, so blocked epics report their tasks as ready. next already has this check — this patch adds the same gate to ready.

With epics fn-1 → fn-2 (depends_on_epics), fn-1 not done:

• Before: flowctl ready --epic fn-2 returns root tasks as ready
• After: returns empty with epic_blocked_by: [fn-1]

Anyway, thanks for everything. This is what the robots are saying about it:

cmd_ready only checked task-level depends_on when determining which tasks are ready. It ignored the epic's own depends_on_epics field, so tasks in blocked epics were reported as ready to work on.

cmd_next already handles this correctly (lines 4425-4441) by filtering out epics whose depends_on_epics aren't all done. This adds the same gate to cmd_ready: if the epic is blocked by unfinished prerequisite epics, return empty ready/in_progress/blocked lists with a new epic_blocked_by field in JSON output.

The bug is latent in flow-next's default workflow because Ralph uses next exclusively and flow-next-work only calls ready on pre-validated epics. External consumers that call ready per-epic independently hit it immediately.