docs(#2171): record the statusline data-source scope boundary (#2178)

Records the statusline scope boundary decided during triage of #2160-2164:
the statusline sources only local, read-only data (refine-existing + new-local),
never credentials or external/network APIs. #2164 (account-usage segment) is
out of scope on this boundary; #2163 (git) is in-scope but on the feature track;
#2160/2161/2162 are approved enhancements.

- docs/adr/2164-statusline-scope-boundary.md (new ADR, Accepted)
- docs/adr/README.md (index row)
- CONTEXT.md (### Statusline glossary/seam entry)
- .out-of-scope/statusline-account-usage.md (#2164 rejection record)

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

docs/adr/README.md

@@ -67,6 +67,7 @@ See **[CONTRIBUTING.md — "Proposing an ADR or PRD"](../../CONTRIBUTING.md#prop
 | [1990-existing-code-onboarding.md](1990-existing-code-onboarding.md) | Existing Code Onboarding Module owns deterministic repo-state detection and onboarding route selection | Proposed |
 | [2121-phase-identifier-parsing-consolidation.md](2121-phase-identifier-parsing-consolidation.md) | Phase-identifier parsing consolidation — single canonical owner (phase-id.cts) + anti-divergence guard | Accepted |
 | [2143-markdown-table-and-mutation-consolidation.md](2143-markdown-table-and-mutation-consolidation.md) | Markdown table model, bounded mutation, and fail-loud consolidation (#1372 part 2) | Accepted |
+| [2164-statusline-scope-boundary.md](2164-statusline-scope-boundary.md) | Statusline draws its data boundary at local, read-only sources (no external/credentialed data) | Accepted |
 
 ## Seam map