fix(tui): hint mention depth cap on misses

[cyq1017]

Summary

• add a status hint when path-like @-mention completion misses under a nonzero walk depth
• keep ordinary filename misses unchanged
• keep the default walk depth and completion behavior unchanged

Refs #2488

Verification

• cargo test -p codewhale-tui try_autocomplete_file_mention_no_match --locked
• cargo fmt --all -- --check
• git diff --check

Greptile Summary

This PR refines the @-mention autocomplete miss experience by showing a contextual depth-limit hint only when the typed path is plausibly too deep for the current mention_walk_depth setting.

• Adds no_file_mention_matches_status and path_partial_reaches_walk_depth helpers to file_mention.rs; the hint fires only when the component count of the partial is at or above walk_depth, and is suppressed entirely when walk_depth == 0 (unlimited).
• Adds three targeted tests: deep-path hint fires, shallow-path hint suppressed, and unlimited-depth hint suppressed — the last of which directly covers the previously-flagged missing negative case.

Confidence Score: 5/5

The change is narrowly scoped to status-message text generation and has no effect on autocomplete logic, file I/O, or app state beyond status_message.

Both helpers are pure functions with no side effects. The guard for walk_depth == 0 and the component-count comparison are straightforward. Previous review feedback (depth-component counting and the missing unlimited-depth negative test) has been fully addressed. The three new tests cover all meaningful branches.

No files require special attention.

Important Files Changed

Filename	Overview
crates/tui/src/tui/file_mention.rs	Extracts status-message logic into two well-scoped private helpers; handles walk_depth==0, cross-platform path separators, and empty components correctly.
crates/tui/src/tui/ui/tests.rs	Adds three new unit tests covering the positive case (hint fires for deep partial), negative case (shallow partial, no hint), and unlimited-depth case (walk_depth=0, no hint).

Flowchart

%%{init: {'theme': 'neutral'}}%%
flowchart TD
    A[try_autocomplete_file_mention] --> B{candidates empty?}
    B -- Yes --> C[no_file_mention_matches_status\npartial, walk_depth]
    C --> D{path_partial_reaches_walk_depth?}
    D -- walk_depth == 0 --> E["No files match @{partial}"]
    D -- component_count < walk_depth --> E
    D -- component_count >= walk_depth --> F["No files match @{partial}\n(mention_walk_depth=N; use /config set\nmention_walk_depth 0 to search deeper)"]
    B -- No, single match --> G[replace + attach]
    B -- No, multiple --> H{shared prefix longer?}
    H -- Yes --> I[extend partial]
    H -- No --> J[show top 5 candidates]

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_{Reviews (2): Last reviewed commit: "fix(tui): narrow mention depth hint" | Re-trigger Greptile}

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[Hmbown]

Harvested into #2504 for the v0.8.50 release slice.

What landed:

• a97675824 / 29f57665e add the targeted mention_walk_depth status hint for deep path-like @ misses while keeping autocomplete behavior unchanged.

Local verification on the harvest branch:

• cargo test -p codewhale-tui --all-features --locked try_autocomplete_file_mention_no_match -- --nocapture
• cargo clippy -p codewhale-tui --all-targets --all-features --locked -- -D warnings
• ./scripts/release/check-versions.sh

Live #2504 CI is green across Ubuntu, macOS, Windows, lint, version drift, CodeQL, mobile smoke, npm smoke, and GitGuardian. Thanks @cyq1017; I kept your authorship on the harvested commits.