feat(server): report the generators that draw and rotate credentials

[JeroenSoeters]

Summary

A generator draws a credential and, when it declares a cadence, rotates it
unattended. Nothing in the conversation could see them, so the questions people
actually ask about a rotating credential had no answer short of dropping to the
CLI: what rotates, when did it last, what takes its value.

list_generators answers those. It reports each generator's cadence, the
instant of its last committed rotation, and the resources bound to it. The drawn
value is not among them and cannot be — only the generation's identity and its
destinations are projected — so asking what rotates never risks printing a
secret.

Two decisions worth reviewing

Not version-gated, unlike the policy tools. Those gates ask the local
formae binary its version, but what decides whether generators exist is the
agent. An agent predating the feature has none, so its 404 is answered as an
empty collection: the truthful answer rather than a capability complaint. That
also keeps the tool working against a hosted installation, where there may be no
local binary to ask.

A new skill rather than an addition to the authoring front door. That front
door is explicitly a thin dispatcher that hands deep procedures to focused
skills, so it gains one branch pointing at the new one. It is also deliberately
not folded into the policy skill: a policy governs a stack, a generator is
referenced by the properties that take its value, and those were separated in
the design for exactly that reason.

The skill carries the constraints that actually bite rather than a tour of the
feature: every destination of a drawing generator has to be in the same apply,
the cadence floor is one minute, and omitting a cadence draws once and never
rotates — which is the direct replacement for a seed pinned with setOnce.

One thing stated plainly rather than glossed

A resource consuming a generator-fed secret by reference does not yet follow a
rotation: the secret is updated and the referencing resource is not, until the
next ordinary apply. The skill says so, in the terms a user would feel it —
rotating a database password leaves the database expecting the old one — so that
nobody promises rotation that leaves the consuming side stale.

Tests

list_generators is registered in the shared collection matrix, so it inherits
the whole contract those endpoints are held to rather than getting its own
partial version. Its tool tests assert the cadence and the bound destination as
well as the label, because a projection that dropped either would still contain
the label.