chore: v0.1.15.dev5 dev release

[xeophon]

Summary

• Bump verifiers.__version__ to 0.1.15.dev5.
• Add assets/release/RELEASE_v0.1.15.dev5.md with notes covering changes since v0.1.15.dev4.
• Summarize the bundled vf.Terminus2 harness and explicit Harbor sandbox defaults from Add Terminus2 harness #1356 in this dev cut.

Testing

• uv run ruff format verifiers/__init__.py
• uv run ruff check --fix verifiers/__init__.py
• uv run pre-commit run --files verifiers/__init__.py assets/release/RELEASE_v0.1.15.dev5.md
• uv run python3 -c 'import verifiers; print(verifiers.__version__)'\n- uv build\n- git push -u origin chore/v0.1.15.dev5-dev-release (pre-push hooks: ruff check, ruff format, Semgrep v1 policy, Sync AGENTS.md from docs, ty)

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Note

Low Risk
Release-only change: bumps the package version and adds release notes; no functional code paths are modified in this PR.

Overview
Updates verifiers.__version__ from 0.1.15.dev4 to 0.1.15.dev5.

Adds RELEASE_v0.1.15.dev5.md documenting highlights for this dev cut (notably the bundled vf.Terminus2 v1 harness and explicit Harbor sandbox default behavior) and linking to the full changelog.

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