Move context window construction (build_context_window, plan_context,
render_journal_text, assemble_context), token counting, error
classification, and related helpers from agent.rs into context.rs.
All extracted functions are pure — they take inputs and return values
with no mutable state access. State mutation stays in agent.rs
(compact, restore_from_log, load_startup_journal).
agent.rs: 1504 → 987 lines (-517)
context.rs: 365 lines (new)
Net: -152 lines (duplicate comments removed)
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
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
