docs: reflect recently-shipped features (#65-#70)

[fentas]

Summary

Catches the docs up with the last 6 merged PRs.

`docs/llm.md`

• Chat surfaces section — Alt+C inline panel vs. Alt+Shift+C full overlay (same conversation ring, mutually exclusive cursor focus).
• Keybindings section — every shipped LLM binding + Alt+H cheat-sheet pointer.
• Configuration reference split into sub-sections (Core / Chat surfaces / Persistence / Visual signals / Buffer sizes / Model struct). Adds rows for: `models`, `inline_chat_rows`, `overlay_open_policy`, `chat_persist_enabled`, `chat_persist_path`, `chat_persist_max_bytes`, `history_turns_max`, `max_turn_bytes`, `dialog_system_prompt`, `dialog_parse_retry_max`.
• Documents the `Model` struct with the per-model `history_turns_max` trim knob (feat(llm): Model struct — name + per-model knobs as one unit #66) and a multi-model example.

`docs/modules.md`

• Hook list gains `default_bindings`, `onResize`, `isInlineChatActive`, `extraReserveRows`.
• default_bindings section — module-owned keymap pattern (feat(dispatch,llm): modules register their own default_bindings #70: dispatcher concat + user-overrides-win precedence).
• extraReserveRows section — inline-panel reservation contract used by the Alt+C panel (feat(llm): inline chat panel (Alt+C) above the statusbar #64) + the related `isInlineChatActive` getter.

Test plan

• Markdown renders cleanly (verified via inspection)
• `zig fmt --check` clean (no code changes)
• Read docs through once more after merge

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