perf: optimize shell startup with batched PATH operations and Homebrew caching

- Batch Fish PATH additions into single operation (reduces startup by ~13ms)
- Cache Homebrew shellenv output for 7 days (reduces startup by ~14ms)
- Update CLAUDE.md with TODO list update workflow guidelines
- Change default chezmoi cd shell from bash to fish
- Remove redundant bun completion sourcing in bash

Performance improvements measured through shell startup profiling.

home/.chezmoi.toml.tmpl

@@ -73,5 +73,5 @@ args = [
 	command = "nu"
 {{ else }}
 [cd]
-	command = "bash"
+	command = "fish"
 {{ end }}