Document the semantic boundaries of context management as called for
in the agent-refactor README (suggested document split, item 5):
- context window region definitions and history budget formula
- ContextWindow vs MaxTokens distinction
- session history contents (no system prompt stored)
- Turn as the atomic compression unit (#1316)
- three compression paths and their ordering
- token estimation approach and its limitations
- interface boundaries between budget functions and BuildMessages
Also documents known gaps: summarization trigger not using the full
budget formula, heuristic-only token estimation, and reactive retry
not preserving media references.
Ref #1439
xiaoen