Phase 0 entry gate + branch-naming convention fix

Record Phase 0 entry baseline: 820 passing, 2 pre-existing failures (Client.CLI.Tests.SubscribeCommandTests.Execute_PrintsSubscriptionMessage and Tests.MxAccess.MxAccessClientMonitorTests.Monitor_ProbeDataChange_PreventsStaleReconnect), 0 build errors, 167 build warnings. The two failures exist on v2 as of commit 1189dc8 and are unrelated to the rename. Phase 0 exit gate adapts the requirement to "failure count = baseline (2); pass count ≥ baseline (820)".

Branch-naming convention updated in implementation/overview.md and phase-0 doc: cannot use `v2/phase-N-slug` form because git treats `/` as path separator and `v2` already exists as a branch, blocking creation of any `v2/...` branch. Convention is now `phase-N-slug` (no v2/ prefix).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

docs/v2/implementation/phase-0-rename-and-net10.md

@@ -2,7 +2,7 @@
 
 > **Status**: DRAFT — implementation plan for Phase 0 of the v2 build (`plan.md` §6).
 >
-> **Branch**: `v2/phase-0-rename`
+> **Branch**: `phase-0-rename`
 > **Estimated duration**: 3–5 working days
 > **Predecessor**: none (first phase)
 > **Successor**: Phase 1 (`phase-1-configuration-and-admin-scaffold.md`)
@@ -41,7 +41,7 @@ The phase exists as a clean checkpoint: future PRs reference `OtOpcUa` consisten
 
 ## Entry Gate Checklist
 
-Verify all before opening the `v2/phase-0-rename` branch:
+Verify all before opening the `phase-0-rename` branch:
 
 - [ ] `v2` branch is at commit `a59ad2e` or later (decisions #1–125 captured)
 - [ ] `git status` is clean on `v2`