feat(tui): add mention browser completions

[cyq1017]

Summary:

• add an opt-in mention_menu_behavior = "browser" mode for @-mention completions
• list only immediate children for the typed directory segment, sorted deterministically
• keep the default fuzzy/frecency completion behavior unchanged

Validation:

• cargo fmt --all -- --check
• cargo check -p codewhale-tui --all-features --locked
• cargo test -p codewhale-tui --all-features --locked browser_completions
• cargo test -p codewhale-tui --all-features --locked mention_popup_browser_mode_lists_immediate_directory_children
• cargo test -p codewhale-tui --all-features --locked mention_completion_caps_are_configurable
• cargo test -p codewhale-tui --all-features --locked file_mention
• git diff --check

Refs #2368

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[Hmbown]

Thanks @cyq1017, this is a useful opt-in UX improvement for deep workspaces. I harvested it into #2504 as 73cd72166 and f185d4691 after upstream CI was green.

I also added a small local containment hardening while resolving the release-branch conflict: browser mode now rejects parent/root/prefix path components in the directory segment, with browser_completions_reject_path_escape_segments covering the @../ case.

Local verification on the release branch:

• for filter in browser_completions mention_popup_browser_mode_lists_immediate_directory_children mention_completion_caps_are_configurable file_mention; do cargo test -p codewhale-tui --all-features --locked "$filter" -- --nocapture || exit 1; done
• cargo check -p codewhale-tui --all-features --locked
• cargo fmt --all -- --check
• git diff --check
• ./scripts/release/check-versions.sh
• cargo clippy -p codewhale-tui --all-targets --all-features --locked -- -D warnings

[Hmbown]

This PR was harvested into v0.8.50. Closing as already landed.