Interestingly, this was tackled in this blog post[0] a month ago. They claim that plan files aren't token-efficient, because after reading the plan the workhorse model then reads all the relevant files anyways.
And I think plan files should focus on general ideas and invariants, not do “implementation as prose”. That way they perform as mini-ADRs that are useful historically, especially to mine why the system is the way it is.
I managed even the orchestrator to NOT read the plan whole, at once, but in sections.
The most useful thing is the task ledger the task agent leaves behind, which alongside its structured status message makes a very resilient handoff between all stages.