fix(tui): harden terminal resume and runtime context

[Hmbown]

Summary

• keep alternate-screen enabled in default auto mode so transcript scrolling stays inside the TUI instead of falling back to terminal scrollback; inline scrollback now requires explicit --no-alt-screen or tui.alternate_screen = "never"
• queue ResumeEvents if the terminal guard drops while the engine event channel is full, so mouse capture/raw terminal state is restored after interactive tool cancellation or bursts
• scope crash-checkpoint recovery to the resolved launch workspace instead of the shell cwd, so --workspace cannot recover an old tree by accident
• add deepseek_version to the system prompt Environment block and tell agents to distinguish runtime version from a stale workspace Cargo.toml

Test plan

• cargo test -p deepseek-tui --bin deepseek-tui terminal_mode_tests --locked
• cargo test -p deepseek-tui --bin deepseek-tui terminal_guard_queues_resume_when_event_channel_is_full --locked
• cargo test -p deepseek-tui --bin deepseek-tui render_environment_block_lists_supplied_locale_and_workspace --locked
• cargo test -p deepseek-tui --bin deepseek-tui workspace_scope_ --locked
• cargo fmt --all -- --check
• git diff --check
• cargo clippy -p deepseek-tui --bin deepseek-tui --all-targets --locked -- -D warnings
• cargo build --release --locked -p deepseek-tui-cli -p deepseek-tui

Local smoke

Installed rebuilt target/release/deepseek and target/release/deepseek-tui into ~/.cargo/bin for maintainer terminal testing before merge/release.

[gemini-code-assist]

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