feat(tui): add fuzzy prompt history search

[ZhiXiao-Lin]

Summary

• add /history and Ctrl+R fuzzy search over prompts from the current TUI session
• keep the current draft untouched until Enter or Tab accepts a result, while Esc cancels and repeated Ctrl+R cycles matches
• bound results to 100, retain duplicate history positions, prefer newer equal-score matches, and support keyboard, paste, wheel, and mouse selection
• register and document the command, and add focused ranking, navigation, rendering, and slash-policy coverage

Validation

Validated with the same published A3S dependency boundary used by CI:

• cargo fmt --all -- --check
• cargo clippy --locked --all-targets -- -D warnings
• cargo test --locked --all-targets
• CARGO_PROFILE_RELEASE_LTO=false cargo build --locked --release

[ZhiXiao-Lin]

Integrated through #40 at merge commit c252583e413f70a4cb265012d776f9d050b94de5. The consolidated v0.9.7 branch contains this PR's latest head e63e8e2dfce54fe8729397b2767f4ac1dd0fcf9e and passed Linux, macOS, and Windows CI.