chore(lint): dedupe tree-sitter cfg gate; collapse nested else-if

[ajianaz]

Fixes #532

What

• Remove the redundant inner #![cfg(feature = "tree-sitter")] from src/index/ast.rs — the mod ast; declaration in src/index/mod.rs is already cfg-gated, so the file can never be compiled without the feature anyway (finding Core CLI structure and commands (review, scan, init, auth, hook) #1 of clippy: two pre-existing -D warnings failures (block local validation with strict clippy) #532)
• Collapse the else { if ... } wrapper in the Brain-search output block of src/main.rs into else if ... else (finding OpenAI-compatible LLM engine (reqwest-based, streaming status) #2 of clippy: two pre-existing -D warnings failures (block local validation with strict clippy) #532, at its current location after the fix(dead-code): resolve cross-crate method calls + skip public API by default #529/feat(review): inject enclosing control-flow scope into review prompts #531 refactors shifted it)
• The second else { if !opts.quiet { ... } } site (~line 1822) is intentionally NOT collapsed: the block continues with a statement after the inner if, so collapsing would change semantics. Current clippy correctly does not flag it.

Why

#532 reported two pre-existing cargo clippy --all-targets -- -D warnings findings that block contributors on stricter toolchains than CI's. Verification on rustc 1.97.1 showed both already lint-clean (see issue comment), so this PR applies the cleanups defensively as suggested in the issue — making the source shape match the lint bar on every toolchain, not just the current one.

Testing

• cargo fmt --all -- --check — clean
• cargo clippy --all-targets --features tree-sitter -- -D warnings — 0 warnings
• cargo clippy --all-targets -- -D warnings — clean
• cargo test --features tree-sitter — 926 passed (904 unit + 16 CLI + 6 config), 0 failed

No behavior change: cfg removal only affects a file already feature-gated at its mod declaration; the else-if collapse is the standard clippy-validated transform.

All commits signed-off (DCO).