refactor: extract chat state owner

[zatevakhin]

change

• extract the exact 23 chat fields into private ChatState
• move ElicitationUiState and chat-local transitions into the owner
• preserve acp, handler, config, runtime, and ratatui coordination at their existing boundaries
• add focused owner coverage for defaults, reset, fork, prompt, cancel, timing, undo, elicitation, streaming, replay, and tool reconciliation

validation

• cargo fmt --all -- --check
• cargo check --all-targets --all-features
• cargo clippy --all-targets --all-features -- -D warnings
• cargo build --all-targets --all-features
• cargo test --all-targets --all-features (886 passed, 0 failed, 0 ignored)
• git diff --check

boundary

• exact base: 81e231e7d99f0ccc8fd73c20bb2d81956d4d3677
• exact 10-file allowlist
• one private mod chat_state; and one pub(crate) chat: ChatState
• 23 owner fields; app reduced from 40 to 18 direct fields
• no render state, phase 5, protocol, transport, persistence schema, or cache architecture changes
• four phase 11 compatibility forward tags retained

integration, manual tui smoke, renderstate, and phase 5 remain pending.