fix(acp): report model context window

[yan-ad]

Summary

• Thread model context-window metadata through selectable model entries
• Have ACP resolve context window from the loaded selectable catalog before falling back to discovery/custom provider lookup
• Preserve effective-catalog snapshots with optional context-window metadata

Why

ACP could report the default 1.5k context for providers/models whose real context was already known by the model catalog, because that metadata was dropped before ACP session configuration. This affects providers generally.

Validation

• cargo fmt
• cargo test acp

[yan-ad]

Superseded by #44. The fix/acp-context-window commits were merged into that branch in merge commit 0062927, reconciled with current main, documented in _docs/acp.mdx, and validated with the ACP/model-catalog test suites. Closing this PR so review can continue in #44.