Epic: #1975

OpenAI and Claude providers both implement context_window() with long match arms
for known model prefixes. Extract a shared lookup table.

pub(crate) fn known_context_window(provider: &str, model: &str) -> Option<usize>

Locations: openai/mod.rs:369, claude/mod.rs:770

Acceptance Criteria:

• Shared lookup function
• Both providers delegate to it
• Easy to add new models

Complexity: Small | Effort: 1h
Ref: DRY-07