Adds a generic plugin-declared selectable-context-window surface mirroring thinkingLevels: ModelCatalogEntry.contextWindows + contextWindowDefault through catalog normalization and the gateway protocol, session validation on sessions.create/patch, and a 200K/1M switch inside the Control UI model picker for Claude CLI 5-series models. The Anthropic plugin owns the option mapping: explicit 1m → `[1m]` argv suffix, 200k → bare id + CLAUDE_CODE_DISABLE_1M_CONTEXT=1, omitted → bare id (shipped default argv). Run budgets follow the selection on both CLI and native paths, so a 200K session gets a matching auto-compact window instead of a silent 1M budget.
Review fixes landed in this PR: run-owner prepared-fact plumbing so ordinary replies honor the selection; atomic catalog overlay merge and normalization for the options/default tuple; one-owner tuple reads in the picker; sessions.create key-presence patch semantics; native-run budget capping.
Feature direction and in-picker switch shape by @obviyus (maintainer review).
Ayaan Zaidi