Move context window building functions from agent.rs to context.rs:
- build_context_window, plan_context, render_journal_text, assemble_context
- truncate_at_section, find_journal_cutoff, msg_token_count_fn
- model_context_window, context_budget_tokens
- is_context_overflow, is_stream_error, msg_token_count
Also moved ContextPlan struct to types.rs.
Net: -307 lines in agent.rs, +232 in context.rs, +62 in types.rs
