fix(tui): highlight session config values only

[CSRessel]

Summary

🤖 Generated with Nori

• Render ACP session-config history assignments as Name= plus a separately highlighted value span.
• Covers startup, update, and option-set history cells so Mode=Default and Model=... leave the name/separator plain.
• Remove the earlier unrelated MCP argument styling change from this PR.

Test Plan

• RUSTC_WRAPPER= cargo test -p nori-tui nori::session_config_history::tests::history_cells_highlight_values_not_names
• RUSTC_WRAPPER= cargo test -p nori-tui
• RUSTC_WRAPPER= cargo build --bin nori
• RUSTC_WRAPPER= cargo test -p tui-pty-e2e
• just fmt
• RUSTC_WRAPPER= just fix -p nori-tui
• RUSTC_WRAPPER= cargo insta pending-snapshots --manifest-path tui/Cargo.toml
• git diff --check
• TUI smoke via tmux with ElizACP config

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