v0.12.0

Fixed

• (agent-ux) address LSP reliability and usability issues reported in 24-commit assessment
• (deps) bump sha2 0.10 → 0.11 with digest 0.11 migration
• (deps) bump tree-sitter 0.25 → 0.26 with parse_with_options migration

Other

• (deps) bump serde_json, ignore, rmcp, and which
• (treesitter) offload CPU-intensive parsing to blocking pool and optimize concurrency
• (lsp) [breaking] replace blocking file reads with async tokio::fs and add benchmarks