fix(trail): let tune rewrites drop cosmetic placeholders

tune --run was skipping most runners with "dropped placeholder(s): {{branch}}": the model legitimately stops naming {{branch}} in prose (the diff is taken against HEAD, and the command only needs {{base_branch}}), and the strict "preserve every placeholder" check rejected the otherwise-good rewrite.

Reframe the check around what's actually unsafe: an ADDED placeholder renders as literal {{junk}} (the backend only substitutes the known set) so it's still a hard reject; a DROPPED placeholder just leaves a substitution slot unused, which is safe — now allowed and surfaced as a "note: <runner> no longer references …" line so the user still sees it in the git diff.