docs(trust-levels): state the all-zero measurement rule covers production software-only records too

[rajnisht7]

What this changes

Closes #240.

runtime.measurement is required on every record. For a software-only platform, the correct value (all-zero digest) was only stated in prose as covering "development records" but a production software-only producer (origin.kind: self, no hardware root of trust) needs to write something too, and today that's only inferable from the example block, not stated by the rule itself.

• docs/trust-levels.md: the sentence under the Level 0 example now covers both development and production software-only records explicitly.
• schema/trace-claim.json + the packaged copy (src/agentrust_trace/schema/trace-v0.2.json, kept in sync per test_packaged_schema_matches_the_normative_schema): extended the measurement field description with the same all-zero-when-software-only note, per the issue's optional suggestion, so the text closest to a validator states the convention too.

Type of change

• Editorial (typo, link fix, clarification: no normative effect)
• Non-breaking spec change (new optional field, new platform profile, informative addition)
• Breaking spec change (requires 14-day comment period and Project Lead sign-off)
• Schema change
• Example addition

Spec section

None

Checklist

• DCO sign-off on all commits (git commit -s)
• CHANGELOG.md updated (for any normative change)
• Breaking changes marked with <!-- CHANGED: #NNN: description --> in spec text
• Backward compatibility statement included (for breaking changes)

[Yatsuiii]

I don't think this can be treated as editorial as written.

The released v0.9.0 reference implementation intentionally emits non-zero runtime.measurement values with runtime.platform: "software-only":

• TraceAGTAdapter sets the platform to software-only and uses sha256(merkle_chain_tip).
• TraceSandboxAdapter defines the unattested measurement as sha256(image_digest + "\n" + bundle_hash) and emits it under software-only.
• The released tests pin both behaviors: AGT chain-tip measurement and sandbox image/policy binding.

That contradicts the changelog statement that existing software-only records already use the all-zero value. The proposed prose would declare released first-party producer output incorrect, while the unchanged schema/model would continue accepting it, creating a new documentation/validation disagreement.

The underlying question appears to be what runtime.measurement means when there is no hardware root:

1. If software-only profiles may carry profile-defined software commitments, their preimage semantics need to be identifiable and documented.
2. If the field is reserved as an all-zero no-hardware sentinel, the adapters need a migration path and somewhere else to carry their current commitments; that is not an editorial-only change.

I think this PR should pause and #240 should be re-scoped around that semantic choice before the all-zero rule is stated as universal.