implement skill should verify it's on an appropriate feature branch before starting work

[JGormanAlphaSimplex]

Right now the implement skill just says "Commit your work to the current branch," with no check on what that branch actually is. In practice this means it'll happily start implementing work -- and committing -- straight onto main/master, or onto some unrelated branch left over from a previous task.

I'd like implement to add a quick branch sanity-check as its first step, with two rules:

1. Hard block on trunk branches -- if the current branch is main or master, stop and ask the user to create/switch to a feature branch before doing any implementation work.
2. Soft check on relevance -- if the branch isn't a trunk branch but its name looks clearly unrelated to the spec/tickets being implemented (e.g. branch fix-typo while implementing "add OAuth support"), flag this to the user and ask them to confirm or switch, rather than silently proceeding. This should be a loose judgment call, not a strict naming-convention check -- only flag obvious mismatches, not just imperfect names.

Suggested addition to skills/engineering/implement/SKILL.md, right after the frontmatter:

Before making any changes, check the current git branch:
- If it's `main` or `master`, stop and ask the user to create or switch to an appropriate feature branch first.
- If the branch name looks clearly unrelated to the spec/tickets being implemented, flag the mismatch to the user and confirm before proceeding.

Happy to be told this is better handled elsewhere (e.g. a shared pre-flight check reused across skills) -- just flagging the gap as I hit it in implement specifically.