Modernize: sed (stream editor)

[martinciu]

Evaluate replacing vanilla sed with a modern alternative for interactive use. Keep sed for scripts and portability.

Candidate (re-evaluate at decision time)

• sd — saner regex/replace syntax, no escaping / etc.

Decide

• Don't alias over sed; introduce alongside.
• If adopted: add to Brewfile, update CLAUDE.md and shell-colors cheatsheet.

Out of scope

• Don't bundle multiple tool swaps into one PR.