Meta: rework bot commands & interactions

[xmo-odoo]

"Extended" commands set has issues, the priorities are a bit too non-orthogonal and leads to limited use-cases for p=1

Redo the entire commands set, to have more orthogonal and precise commands, which are easier to use and clearer:

• remove all fuzz, if a command is unknown the comment is an error and the commenter can fuck off
• remove numerical priorities, though maybe keep an "emergency" command which has non-trivial expansion
• allow bypassing (non-overridable) CIs when goal is to get something merged quickly, but not so much that it's an emergency e.g. get a fix for a blocking bug that's made its way into the target branch
• allow prioritizing PR for the next staging, but without isolating that PR or cancelling existing staging, just makes sure the PR is selected for the next staging (if otherwise suitable)
• support attaching justifications to commands somehow, possibly require it (?)
  that's mainly for override but it could be useful for some other stuff as well
• remove current version of skipci?
• what happens when down-prioritising one PR of a batch where all are prioritised? (maybe reifying batches and moving prioritisation there? cf Mergebot 2 #789, such reification might also be useful or even necessary for Make staging / batch commits more easily accessible #768)

Orthogonal bits from current priority, which might be exclusive:

• skip CI (non-overridable)
• skip CI (all)
• prioritise for staging (don't stage alone but really should be in next)
• stage alone (ish)
• cancel current staging (now? on every retry?)

Also

Retrying an emergency PR which failed should probably cancel the current staging if any.

Related:

• Forward-port without even merging #418
• Rework commands parsing and execution entirely #507
• Allow updating PR limit of FP sequences #506
• Add limited r+ allowance on fp if author #511
• Add a way to get a PR in the next staging without staging it "alone" #662
• Allow prioritising waiting PRs over splits #798