agent-governance-toolkit-core[migrate] extra pins an unresolvable agt-policies range

[karimad]

Context

Follow-up from #4017 (draft PR moving agt-policies from a base dependency
of agent-governance-toolkit-core to an opt-in migrate extra, so the base
package and [full] install again without an unpublished transitive pin).

The remaining gap

The migrate extra itself still pins:

migrate = [
    "agt-policies>=5.1.0,<6.0",
]

This range predates #4017 and was not changed by it — agt-policies>=5.1.0
is not published on PyPI yet (latest is 5.0.0), so pip install agent-governance-toolkit-core[migrate] is unresolvable today.

It gets worse once agt-policies>=5.1.0 is published: that release pins
agent-control-specification>=0.4.0b0,<0.5.0, which does not overlap
agent-governance-toolkit-core's own base pin of
agent-control-specification>=0.3.1b0,<0.4.0 (added in #4017 so agent_os's
direct ACS imports keep working). Installing [migrate] would then require
two mutually exclusive ACS ranges at once.

This is the same class of bug as #3414 and #3733 — a package pinning an
unpublished (or, here, soon-to-conflict) transitive dependency floor — this
time recurring one level down inside an opt-in extra rather than the base
install.

Suggested fix

Once an agent-control-specification release exists that both
agent-governance-toolkit-core and agt-policies can share, widen the base
ACS pin (or narrow agt-policies's pin) so [migrate] resolves again. Until
then, this is worth tracking so it isn't rediscovered as a fresh "pip install
is broken" report.