feat(tui): render initial config-options banner at session startup

[CSRessel]

Summary

🤖 Generated with Nori

• Show an explicit banner the first time the agent announces its session config options at startup:
  `• Claude Code options: Mode=Default, Model=Opus 4.6, Effort=High (/config to change)`
• Replaces the previous baseline-only behavior (feat(tui): Make agent option history user-facing #480) which silently swallowed the initial `SessionConfigUpdate` and left no on-screen indication of the agent's options or how to change them.
• Subsequent updates continue to render only changed values (no behavior change after the banner).

Pairs with #480 (config-options diff rendering) and #481 (mode-changed cell) — finishes the cleanup of the original `ACP config options updated: …` jargon line.

Test Plan

• New unit test `session_config_update_history_renders_startup_banner_on_first_snapshot` (snapshot in `session_config_update_startup_banner.snap`)
• Existing `session_config_update_history_shows_only_changed_values_after_baseline` updated to drain the new banner and still snapshot just the diff
• `cargo test -p nori-tui` — 1304 passed
• `just fmt` + `just fix -p nori-tui` — clean

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