Clarify generated git handoff merge instructions

[darlingm]

Summary

• Replace the command-shaped merge_and_process <sender> <commit> payload with a direct natural-language merge instruction.
• Keep Git mechanics and conflict handling with the receiving role rather than introducing a new helper or mandatory shell command.
• Update the shared handoff rules, protocol examples, generated-payload assertions, and queue fixtures.
• Assert that generated handoffs no longer contain merge_and_process.

Generated payloads now say:

Merge commit <commit> from role <sender> into your current branch. Then process the merged state according to your role and constitution.

Fix 1 of 2 for #29. The companion change is #31 on six-pack, which has one project-local reference to the same pseudo-command.

Test plan

• bb test — 24 tests, 92 assertions, 0 failures, 0 errors