Update flow_template reference page: add 'decision' attestation type

[dangrondahl]

Problem

The Flow Template reference page (template-reference/flow_template.md, published at https://docs.kosli.com/template-reference/flow_template) documents the attestation type field with a hand-written enum that is out of date. It does not mention the decision attestation type, which is now supported in flow templates by the API (added in kosli-dev/server PR kosli-dev/server#5988).

Current page lists trail-level types as: generic, jira, junit, pull_request, snyk, sonar, * — missing decision.

What to update

• Add decision to the documented trail-level attestation type values (and confirm whether it is also valid at the artifact level — verify against the API schema / server model src/model/flow_template.py).
• Briefly explain what a decision attestation requirement means in a template (it requires a decision attestation on the trail for compliance; a decision is a system attestation referenced at runtime as system:decision but written as the bare decision in templates).
• Review the example YAML and <ParamField> descriptions on the page for any other drift from the current schema.

Dependency / context

This page is currently maintained independently of the JSON Schema. There is a companion issue to generate the static schema files from the API (the source of truth). Once that lands, this reference page should be kept consistent with the generated schema — ideally cross-check this page against schemas/flow-template.json after it is regenerated, so the prose docs and the machine-readable schema agree.

Acceptance criteria

• decision is documented as a valid flow-template attestation type, with a short explanation.
• Artifact-level vs trail-level applicability is verified against the server model and stated correctly.
• Example and field descriptions match the current schema.

References

• Page source: template-reference/flow_template.md
• Server model: kosli-dev/server src/model/flow_template.py (the Attestation.type union now includes Literal["decision"])
• Feature: decision in flow templates — kosli-dev/server#5988 / kosli-dev/server#5962
• Companion issue: generate static schema files from the API