[sudoshi]

Adds .claude/rules/parallel-agents.md and points CLAUDE.md at it as binding on
any change bigger than a single-file edit.

The rule separates two problems that are easy to conflate: worktrees give
isolation, and isolation does nothing for coordination. Layer 2 (decomposition
by domain boundary, the single-writer file list, freezing contracts before
fanning out) and Layer 3 (sequential rebase-and-merge, the green gate, never
letting an agent auto-resolve another agent's conflict) are where the expensive
failures actually happen.

This session produced the evidence. Two efforts independently claimed
net.acumenus.nightingale — #101 renaming the patient app in place, #92
building a new foundation tree. Both were internally correct, neither could see
the other, and git merge-tree reported zero conflicts while producing two
applications competing for one App Store record. It was caught by reading diffs
by hand, after one had already shipped to TestFlight.

Two of the rule's specifics were also violated during this session, which is
part of why it is worth writing down:

• .github/workflows/ci.yml and docs/hummingbird/api-contract/** are
  single-writer files. Both were edited directly rather than proposed.
• "Merge one branch at a time" — five PRs ended up open simultaneously.

Related: #111 adds verify-app-identity-uniqueness.sh, the CI guard that turns
the identity half of this rule into something enforced rather than remembered.