fix(discovery): ignore unrelated JSON type fields

[schickling-assistant]

Problem

Folder discovery treated any JSON type field as an Agent Spec marker. An ordinary ESM package.json with "type":"module" became a phantom agent and could receive st2 state inside the project directory.

Goal

Recognize a standalone type marker only for the supported Agent Spec value, while preserving validation for files with other agent-shaped markers.

Decisions

Narrow the existing candidate predicate instead of adding filename-specific exclusions. This keeps declaration classification semantic across JSON, TOML, and KDL.

Verification

• CI=1 nix develop -c cargo test -p agent-spec only_service_type_is_a_spec_candidate_by_itself --lib — 1 passed
• CI=1 nix develop -c cargo test --test validate type_batch_is_retired_and_flagged_unknown — 1 passed
• nix flake check --print-build-logs --no-write-lock-file — passed all 16 Linux checks

Complexity

No new complexity. One predicate now checks the supported value rather than field presence.

Concerns

A file with another strong Agent Spec marker and an unknown type remains a candidate, so validation still reports the unknown type.

Friction & bottlenecks

Local Nix tracing emitted an ignored permissions warning. Validation itself passed.

Follow-ups

None.

References

Closes #247

Posted on behalf of @schickling

field	value
agent_identity	dev3.direct.omp.vjwqaz7t
session	dev3.vjwqaz7t
agent_persona	generalist
agent_supervisor	unavailable
agent_tool	OMP
agent_tool_version	18.0.11
agent_runtime	OMP 18.0.11
tooling_profile	dotfiles@000f2b3