[Improvement] Add AGENTS.md + SECURITY.md to make the security model discoverable

[potiuk]

This is a proposal for the DolphinScheduler PMC to review — please
correct, reject, or discuss as needed. The maintainer is the
decision-maker; nothing here is a requirement.

This adds the conventional AGENTS.md → SECURITY.md discoverability
chain so an automated security scanner can mechanically locate the
project's existing security model. It does not add or change any
model content — your security model at
docs/docs/en/contribute/join/security-model.md is untouched; this just
makes it reachable via the standard chain.

• SECURITY.md (new) — routes vulnerability reports to the ASF security
  process and points at the existing security model.
• AGENTS.md (new) — a Security section routing agents along
  AGENTS.md → SECURITY.md → your security model.

Context: the ASF Security team is preparing the project for an automated
agentic security scan being piloted by the team. Such scans refuse to run
unless the model is discoverable by that conventional path —
discoverability is the one hard gate. No project source is touched.

Questions / pushback welcome — happy to adjust file placement or wording
to match the project's house style.

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[ruanwenjun]

LGTM, thanks.

[SbloodyS]

+1