chore(rules): forbid flaky tests, time sleeps, and prompt pinning

Adds .sisyphus/rules/test-discipline.md, auto-injected by
rulesInjector hook whenever an agent reads or edits a test file
(**/*.test.ts, __tests__/**, src/testing/**, test-setup.ts,
script/run-ci-tests.ts).

Rules enforced:
- 'bun test' must pass in one process, one run, no isolation flags
- no setTimeout/sleep in test bodies unless time is the SUT
- event tests must subscribe-before-trigger with explicit timeout
- no .only/.skip/process-isolation to mask state leaks
- prompt-builder tests must assert behavior, not text snapshots

Also un-ignores .sisyphus/rules/ so project-scope rules can be
checked in while keeping .sisyphus/{draft,run-continuation,...}
ignored.

.gitignore

@@ -1,5 +1,7 @@
 # Dependencies
-.sisyphus/
+.sisyphus/*
+!.sisyphus/rules/
+!.sisyphus/rules/**
 node_modules/
 
 # Build output