docs: Update security-model docs to remove stale die() references after GitLab die() removal

[github-actions]

Bug Description

PR #2779 (commit 2be7fc3) removed the die() helper from .github/skills/project-planning/gitlab/scripts/gitlab.py, making GitLabError the module's single failure mechanism. Two documentation artifacts still describe the old die()-based behavior and were not updated in that commit:

• docs/security/security-model.md:
  • GL-1 threat description: "A raw exception, die() message, or diagnostic embedding the URL, headers, or upstream body could surface the token"
  • GL-4 title and content: "die() Helper Printing Raw Upstream Body" — "The die() helper prints an error string and exits..." and "local configuration failures use die(), which routes through _emit"
• docs/planning/threat-models/hve-core-comprehensive.yaml:
  • Lines ~2106, 2113: description mentioning "die() message"
  • Lines ~2161-2169: title "die() helper printing raw upstream body" and description "die() prints an error string; a raw upstream body could leak secrets"

Expected Behavior

Both documents should describe the current single-mechanism contract: local configuration and validation failures raise GitLabError (not die()), which is caught once at the main() boundary and emitted via _emit(). The GL-4 threat entry and its yaml counterpart should be renamed/reworded to reflect GitLabError-based emission instead of die().

Steps to Reproduce

1. See commit 2be7fc3 (PR refactor(skills)!: remove the GitLab die() helper in favor of GitLabError #2779), which deletes die() from gitlab.py.
2. Compare against docs/security/security-model.md (GL-1, GL-4 sections) and docs/planning/threat-models/hve-core-comprehensive.yaml (~lines 2100-2170), which still reference die() as an active code path.

Additional Context

• Code: .github/skills/project-planning/gitlab/scripts/gitlab.py
• Docs: docs/security/security-model.md, docs/planning/threat-models/hve-core-comprehensive.yaml
• PR refactor(skills)!: remove the GitLab die() helper in favor of GitLabError #2779 description also notes: "The path:line references in [security] Architectural: introduce central output sink + LOGGER + typed error class to jira/gitlab skills #1555 are stale... Worth correcting the issue body on close." — this issue is a related but distinct doc-drift item scoped to the security-model threat descriptions.

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