Commit the Talos installer schematic

[kelchm]

Summary

talos/talconfig.yaml pins factory.talos.dev/installer/7c9e329e408b058e4bdb5e18e4af75eed95d5b53944d6024fa59f8fa235e1741 on all three nodes, but the schematic that produced that hash exists nowhere in git — docs/architecture.md mentions iscsi-tools/util-linux-tools in prose only. Rebuilding or auditing the node image requires out-of-repo knowledge.

Proposed work

• Commit talos/schematic.yaml; deterministically verify it regenerates the pinned ID (talhelper can compute the ID offline via --offline-mode).
• Document the rebuild/audit path in the talos README or architecture doc.

Note: no Renovate wiring — schematic IDs are content-derived, so there is no upstream release for Renovate to track.

Acceptance criteria

• Schematic in git; recomputing its ID matches the hash pinned in talconfig.yaml (or the delta is documented).
• Rebuild-from-scratch path documented.