fix(tui): reset displayed cost on clear

[LING71671]

Summary

• Reset sub-agent cost accumulators when /clear resets the conversation session.
• Reset displayed cost high-water marks so the footer/sidebar visible total returns to zero after /clear.
• Extend /clear telemetry coverage to assert sub-agent cost, high-water state, and displayed_session_cost() are zeroed.

Closes #1788

Verification

• cargo fmt --all -- --check
• git diff --check
• cargo test -p deepseek-tui --bin deepseek-tui test_clear_resets_all_state
• cargo test -p deepseek-tui --bin deepseek-tui clear_resets_session_telemetry

Notes

• cargo clippy --workspace --all-targets --all-features -- -D warnings was attempted, but current upstream/main fails on pre-existing warnings unrelated to this change: dead-code warnings in crates/tui/src/schema_migration.rs and clippy::needless_return in crates/tui/src/tui/ui.rs.

[gemini-code-assist]

Code Review

This pull request expands the session clearing logic in the TUI to include subagent costs and cost high-water marks, along with updated test coverage. The review feedback points out that certain telemetry fields are still missing from the reset process and suggests including them. Furthermore, it recommends replacing manual field assignments with Default::default() to ensure the session state is fully and maintainably reset.

[Hmbown]

Thanks for the /clear cost-display fix. This was harvested into v0.8.40 release PR #1823 as commit d065241, and #1823 is now CI-green. Closing as superseded by the release branch; thank you for the clear polish fix.

[LING71671]

Thanks for harvesting this into v0.8.40 and for the release-branch closeout.