On page 110 of Codex, where it goes in detail about Task, it lists couple of different examples of dealing with a sentry/guard:
"To capture the sentry: “I sneak up behind him, muzzle him and choke
off his windpipe.” Test Stealthy plus Brawling ( or Martial Arts) plus appropriate wises.
To follow him back to his observation post: “I keep my distance and
trail him on his rounds. Where does he go?” Stealthy plus Observation
or Tracking, maybe even Hunting.
To murder him: ''I hurl my knife into his neck, slicing his windpipe,
killing him before be can make a sound." Throwing plus Stealthy, Knives
and any appropriate wises."
So, these examples are bit confusing. Are these all Linked tests? The confusing part is the “plus” between them, which seems like you would add them together. Also it also uses “plus” -wise skills but that actually would just mean forking them, right?
One underlying approach of BW is to reduce multiple steps to the single key point of drama, so pick a skill that represents the significant part of the plan then fork in others as appropriate.
Linked tests have more the feel of longer planning/working: for example, identify all the conspirators and then kill them has a point of real interest in whether the conspirators are identified properly and a point for whether they are killed and the first affects the second.
When re-reading through it, it does make sense that it’s forking, but aren’t fork skills supposed to be suggested by players, not explicitly requested by GM?
Yes, but I would argue, that they are also already suggested from the players through the fiction in the examples.
In other words:
If you want to FoRK something, you also have to show in the fiction, how you do it.
If you show in the fiction how you do something, the GM might ask you, to FoRK it.
There are two different issues here: whether anyone other than the player can suggest a FoRK applies; and whether the GM can mandate that a player FoRKs a skill.
Unlike some systems where there is a short list of skills, each of which covers a clear set of actions, BW has a vast list of skills that overlap in different ways (the Right Skill, Right Time award even suggests finding new and unexpected applications for a skill is encouraged rather than frowned upon). So, while the abstract task mechanism is player states intent and offers skill, GM accepts/rejects the skill, there will be times when a player might know what they want to do but not be certain what skill (or skills) that approach is mechanically because they aren’t their character. Thus, I see no issue with other participants suggesting “that sounds like X” or “if you want to do surgery, you’ll need good light so could FoRK Chandler”.
However, while the GM has to agree a skill applies, it is clear that it is the player’s choice whether to roll or go with a different plan, so whatever anyone (whether GM or other player) says it sounds like is being FoRKed, the player doesn’t have to make those potentially related skills a non-trivial part of the final plan. Therefore, a GM can never require a skill be FoRKed.
A related area is how FoRKs affect consequences of failure, &c. As @ElvisLiving says, these are suggested through the fiction: so, which skills are FoRKed is also a reflection of where the character is more or less competent/experienced/prepared for the unexpected/blasé/&c. and thus can affect consequence of failure; for example, “you can FoRK Evil-wise as part of your argument to the bishop, but if you fail he will wonder how you know so much about the deals demons make.”