debt(robot-repo-automaton): migrate Rust to Rust/Creusot and retire 6a2 skeleton paths

[hyperpolymath]

Discovery

Found during the 2026-08-30 estate PR-clearance campaign.

robot-repo-automaton is implemented in Rust and its canonical SKELETON
still embeds .machine_readable/6a2/*; the embedded template directory carries
the same legacy paths. The current estate convention is descriptiles, and
Rust components must be Rust/Creusot.

Deferred deliberately

Do not perform the Rust/Creusot conversion or this source migration while
the existing dependency/CI PR backlog is being cleared. This issue records the
departure so it is not lost or accidentally fixed as unrelated churn.

Follow-up scope

• Add the Creusot proof/verification layer to robot-repo-automaton.
• Change canonical skeleton paths from .machine_readable/6a2/* to the
  then-current canonical machine-readable/descriptiles/* form.
• Add a negative regression check rejecting newly generated 6a2 paths and a
  positive control proving descriptiles is emitted.
• Reconcile the generated skeleton with rsr-template-repo and its drift
  check.
• Audit related embedded templates and documentation.
• Only then plan downstream propagation; do not mix it into this PR-clearance
  campaign.

Evidence horizon

Observed in the current local hyperpolymath/gitbot-fleet fresh checkout at
robot-repo-automaton/src/skeleton.rs and
robot-repo-automaton/templates/skeleton/.machine_readable/6a2/. Open and
closed issues were searched for 6a2 descriptiles, Rust Creusot, and
robot-repo-automaton skeleton; no existing issue covered this combined
migration.