Resume/start a task with optional custom instructions

[anshulsao]

Problem

Today flow do <task> either resumes the existing session or
spawns a fresh one — there's no way to inject a one-shot
instruction at resume time. Same for the start path. This
blocks playbook-style automation that wants to nudge a parked
task on a trigger without manually opening the tab and typing.

Proposal

Add an optional instruction payload to flow do:

flow do <task> --with "<instruction>"
flow do <task> --with-file path/to/instruction.md

When provided, flow injects the instruction as the first user
message of the resumed/started session, after the bootstrap
contract runs. Behaves like the user typed it themselves.

Use case — playbook on a parked task

A scheduled playbook fires on some trigger (cron, external
webhook, status change) and wants to do "the thing" on a
specific parked task — e.g. "check if the upstream PR merged
and if so, update the brief and unpark." Today this requires
the user in the loop. With --with, the playbook can:

1. flow do <parked-task> --with "<playbook-specific instructions>"
2. Session resumes, runs the instruction against the task's
   full context (brief, updates, KB), reports back.
3. Playbook closes the loop.

Open questions

• Should --with work on a done task (auto-rollback to
  in-progress like flow update task --status), or require
  explicit unpark first?
• Interaction with --fresh: does --with on a fresh session
  replace or append to the bootstrap?
• How does the trigger source identify itself in the injected
  message? (e.g. prefix with [playbook:<slug>] ...)

[anshulsao]

Implemented in #50 — flow do --with "<instr>" / --with-file <path> (also on flow run playbook). Resolves the open questions: --with on done auto-reopens to in-progress; the instruction comes after the bootstrap prompt; trigger source is marked with a generic [via flow do --with] prefix. Smoke-tested against the real claude CLI (positional prompt arg accepted with both --session-id and --resume). PR has auto-merge enabled.