chore: gitignore .sisyphus/ planning-tool scratch and untrack stale M2 drafts

The previous commit (64f49f8) inadvertently tracked two .sisyphus/ work-order
drafts swept up by `git add -A`. They are planning-tool scratch, were not
authored as plan-of-record, and contradict the re-sequenced plan (they use the
old "M2 = evidence fold" framing and propose a use_event_stream feature gate,
which violates the owner-confirmed Q1 "no fallback flag"). Untracked here
(files kept on disk) and .sisyphus/ added to .gitignore so tool scratch cannot
drift into version control.

Constraint: removes tracking only; files remain locally; no code change
Tested: git status clean after; files still present on disk
Confidence: high
Roadmap-Status: unchanged

Author: johnteee

.gitignore

@@ -12,3 +12,6 @@ build/
 .uv-cache/
 .git/hooks/
 .agents
+
+# Sisyphus planning-tool scratch (not version-controlled; drafts can contradict the plan-of-record)
+.sisyphus/