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>
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