chore: remove planning docs from PR

The spec, plan, research, and notes under
docs/2026-05-21_local-only-format/ guided this change but don't need
to live in the upstream tree. They're preserved in git history on the
earlier commits of this branch (5c43b3d, 15bf65b, 95d2f9d,
ea59785) for anyone who wants the design context.

Coding-Agent: pi
Model: anthropic/claude-opus-4-7
Signed-off-by: Joe Hartshorn <8881940+j-hartshorn@users.noreply.github.com>

Author: j-hartshorn