docs(vrs): add source-linked language index

[myobie]

What changed

• Adds only docs/vrs/ontology.md as a 62-line disambiguation index.
• Links every canonical term to its owning Agent Spec, VRS requirement/vision, or implementation source.
• Separates recurring collisions such as root agent/catalog root, control plane/declared supervisor, presence/session state, identity/bus ID, and message/DING.
• Rejects seat as canonical st2 language and directs authors by meaning, while retaining issue Incubate a transactional content-addressed Agent Spec catalog #52, draft PR prototype: incubate content-addressed catalog admission #55, and the Agent Spec phrase only as leakage evidence.

Why

This supersedes closed PR #36's approach with a deliberately thin index instead of a second behavioral specification. Its source-link invariant is: every canonical term must point to the source that owns its meaning; behavior, lifecycle, ordering, and formats stay in those sources.

Deliberately omitted

• Judge, verdict, and eval-gating behavior.
• Any fleet-wide bare-identity uniqueness promise.
• A simplified DING pipeline or delivery-order model.
• Lifecycle diagrams and behavioral taxonomies.
• CAS admission vocabulary or any attempt to canonize seat.
• Terms without an independently provable authority.

Validation

• PR-triggered Nix flake check — pass.
• git diff --check — pass.
• Local Markdown target and line-range audit — pass after refreshing onto current main.
• Exact Agent Spec commit and issue Incubate a transactional content-addressed Agent Spec catalog #52 / draft PR prototype: incubate content-addressed catalog admission #55 link audit — pass.
• vision.md, requirements.md, and spec.md are byte-identical to current main (SHA-256: 8604467579e097e14f9094cea61ff148cf407d75b90be761e40d7c24bf57e227, 3b50e5da4c21dffdbb531c15d5ebb3ae40fc312ef5ec9400adc051d02bbaacf1, dbdd75cf4b5188379ffbb38ed10c600e0a7042cc32502c5242e93a0163cd30eb).
• cargo test --all-targets passed on the original base. After refreshing onto current main, two runs each reached nomad_survival and hit a different transient Linux ETXTBSY at the same existing binary-replacement line; both exact failing tests passed immediately on rerun.
• Strict axe is unavailable on this host.
• Local cargo fmt --check exposes pre-existing Rust formatting drift with this host toolchain; this PR changes no Rust.

Nathan hand-review required; do not mark ready or merge.

[myobie]

@schickling-assistant can we start thin with this and work up?