Reconcile skill SECURITY.md repo-root links with the cross-artifact portability rule

Discovered while remediating review findings on #2599.

## Problem

Two repository rules now contradict each other.

`hve-builder.instructions.md` (added in #2599) states that a cross-artifact reference must name its target rather than hard-code a path into another package, so references survive extension and plugin packaging.

`skill-security-model.instructions.md` mandates that every skill `SECURITY.md` carry a "See also: repo-wide STRIDE model" callout linking `docs/security/security-model.md`. In practice that is implemented as `../../../../docs/security/security-model.md`, which escapes the skill root and only resolves inside this repository at a known nesting depth.

A skill installed through a plugin or extension package gets a callout pointing at nothing, and the failure is silent.

## Scope

68 skill-root-escaping link instances exist across 15 files under `.github/skills/**`:

| Target | Instances |
|--------------------|-----------|
| `docs/security` | 34 |
| `docs/docusaurus` | 19 |
| `evals/` | 9 |
| `CODE_OF_CONDUCT.md` | 5 |
| `docs/agents` | 1 |

The 34 `docs/security` instances span 11 skill `SECURITY.md` files and exist to satisfy the mandated callout.

## Consequence visible today

PR #2599 converted references in `vex/SECURITY.md` and `customer-card-render/SECURITY.md` but could not remove their mandated callout links, so both files now carry both conventions. In `customer-card-render/SECURITY.md` the converted and unconverted forms sit on adjacent lines of the same References list.

## Options

1. Change `skill-security-model.instructions.md` to require naming the repo-wide model rather than linking it, then convert all 34 instances.
2. Carve out an explicit exception in the portability rule for repository-governance documents that are not part of any package.
3. Publish the security model at a stable URL and reference that instead of a relative path.

Option 2 is the smallest change but weakens the portability rule. Option 1 is the most consistent and the largest diff.

## Acceptance

- The two instructions no longer contradict each other.
- No skill `SECURITY.md` contains both reference conventions.
- The chosen approach is recorded so future SECURITY.md authors are unambiguous.