Add threat model + security-model discoverability (AGENTS.md → SECURITY.md → THREAT_MODEL.md)

[potiuk]

Add a threat model + security-model discoverability for Apache Hop

This PR adds an initial threat model for apache/hop and wires up the
standard discoverability chain so automated security tooling can mechanically
find it:

• THREAT_MODEL.md — a v1 draft threat model authored by the ASF Security
  Team for the Hop PMC to review.
• SECURITY.md — points reporters at the threat model and the ASF security
  reporting process.
• AGENTS.md — a Security section linking SECURITY.md → THREAT_MODEL.md
  so an automated reviewer can follow the chain.

Please read this as a proposal to react to, not a finished document

Following up on the mailing-list thread (thanks @hansva for the go-ahead to
make the initial files): this is a first draft built from public artefacts
only — the README and the repository structure. Almost every security
claim is tagged *(inferred)* and has a matching open question in §14.
The PMC is the decision-maker; please correct, reject, or refine any of
it.

The highest-value things to confirm or correct (all in §14):

1. Hop Server auth posture (Q5) — the crux. What is the default auth on
   the remote-execution REST/web surface? Can a default deployment accept
   unauthenticated remote run requests, or is auth required / is it bound to
   localhost by default?
2. Trust framing (Q1, Q2) — that Hop runs operator-authored pipelines
   (incl. scripting/exec steps) by design and is not a sandbox; the pipeline
   author + operator are trusted.
3. Out-of-scope set (Q3) — website, tests/samples, and bundled third-party
   driver/dependency CVEs.
4. Recurring non-findings (Q18a) — what do scanners most often report
   that you consider non-findings? (e.g. the scripting/exec transforms.)

The point of the model is to let a triager (or an automated scan) classify an
inbound finding as valid / out-of-model / disclaimed-by-design and cite a
section — and to cut false-positive noise (the §11a known-non-findings list
especially).

This is part of an automated agentic security-scan pilot the ASF Security team
is running; a discoverable threat model lets the scan focus on real issues and
suppress the by-design ones.