chore: mark IMPLEMENTATION_PLAN phase 18 complete (#37)

The 0.1.5-beta tag was cut and promoted to stable 0.1.5 after Netclaw
validated the newline-as-statement-separator behavior change.

Author: Aaronontheweb

IMPLEMENTATION_PLAN.md

@@ -265,7 +265,7 @@ bulldoze priorities.
 - [x] `Directory.Build.props` `VersionPrefix` 0.1.4 → 0.1.5,
       `VersionSuffix` → `beta`
 - [x] `RELEASE_NOTES.md` 0.1.5-beta section
-- [ ] Cut 0.1.5-beta tag once branch is merged; promote to stable
+- [x] Cut 0.1.5-beta tag once branch is merged; promote to stable
       0.1.5 after Netclaw validates the behavior change
 
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