chore: make republishing and existing version harder

[notnmeyer]

i think ive mostly shaken out the loose branches. this makes it harder to republish an existing version.

• check for an existing tag as a precondition of the release task.
• get rid of the republish task.

this is mostly to prevent inadvertent changes. gh has separate release immutability settings for later consideration.

➜ task release tag=v0.0.0-rc1
...
task: tag v0.0.0-rc1 already exists on remote
task: precondition not met