Add "guardrails" so phable doesn't change tasks I'm not subscribed/assigned to/whatever by default

Because phab ticket numbers are so long these days, it is easy to copy-paste the wrong thing without noticing. (I managed to re-open a years old task while testing !20 :D) Would be neat to have a "guardrails" option that only allows me to interact with tasks I'm assigned to/subscribed to/are a part of my configured projects/something like that, unless I turn it off via an explicit commandline flag.

No files or tests are named. Start by inspecting the CLI commands that can modify Phabricator tasks and how task membership or project configuration is currently exposed. Define the guard conditions and explicit override behavior; done means unapproved task changes are blocked by default and the override is covered by tests.

Written by the indexing model from the issue text.