Retire file-based coordination (to-do.md, coordination.md, dispatch.sh)

[ms609]

Now that findings and tasks live in this tracker, remove the vestiges of the
dispatcher/queue system so there is one place work is recorded rather than three.

Scope

• Migrate to-do.md's still-open non-findings tasks to issues; delete the file
• Delete dispatch.sh and dev/dispatch/; /next-issue replaces them
• Port the next-issue skill from StratoBayes, adapted to this project
• Decide the fate of coordination.md and completed-tasks.md — both hold
  reasoning that no issue thread carries, so neither should simply be deleted
• Rewrite AGENTS.md for the fork/issues model, including why the fork's
  default branch must be cpp-search and why upstream must only fast-forward
• Sweep dev/expertise/* and remote-jobs.md for stale references

Not in scope: the user-level dispatch skill lives outside this repo and must
be removed by hand.