Add draft project security threat-model document

[potiuk]

Summary

This PR adds an initial draft of a project-level security
threat-model document (draft-THREAT-MODEL.md) so that automated
security scanners running against this repository have a
maintainer-facing reference for which classes of findings are
in-scope vs. out-of-scope for the project.

The document follows the rubric format used by several other ASF
projects piloting improved security-model discoverability for
agentic scanners. Every claim carries a provenance tag:

• (documented) — paraphrased from public artefacts (this repo or
  the project website), cited inline.
• (inferred) — synthesised from code structure or domain
  knowledge; the PMC has not confirmed.
• (maintainer) — confirmed by a CloudStack PMC member in response
  to this draft. (Zero in this initial draft.)

Draft stats:

• ~88 documented claims
• ~64 inferred claims (each maps to a §14 question)
• 38 open questions for maintainers in §14

§14 is the highest-leverage section: answering each question
either promotes one (inferred) tag to (maintainer) or corrects
the underlying claim.

Why "draft-" prefix?

The file is named draft-THREAT-MODEL.md rather than
SECURITY-THREAT-MODEL.md because this is a proposal for the
PMC to review — please correct, reject, or discuss as needed.
Once the PMC ratifies (or substantially edits) the content, the
file can be renamed in a follow-up PR and a discoverability
scaffold (AGENTS.md → SECURITY.md → the model) added so
scanners can mechanically follow the chain.

What this is, and what it is not

This is not a security audit. It is a working triage document
— the reference a triager holds against an inbound report to
decide whether the report is about a CloudStack vulnerability or
about caller misuse / operator misconfiguration / an out-of-scope
concern.

The draft was generated by an automated agentic security scan
being piloted by the ASF Security team; the discoverability work
is independent of any specific scan run.

How to review

1. §14 first. Each answer either confirms one (inferred) tag or
   replaces the inferred claim with the correct one.
2. After that, please skim §3 (out-of-scope) and §13 (triage
   dispositions) — those govern how a vulnerability report would
   be triaged.

Reply edits / corrections inline on the PR, or to the original
security@apache.org thread, whichever fits the PMC's workflow.

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