feat(DEN-1301): add secure offline release-plan HTML

[ORESoftware]

Closed after semantic consolidation

This draft explored a direct Rust --html output path. During review, the existing ready PR #31 was identified as the canonical DEN-1301 implementation because it follows the ticket's intended architecture: the authoritative zed release plan --json model is consumed by a dependency-free renderer, preserving every existing CLI and release-planning path.

The useful unique hardening from this draft was transferred to PR #31 before closure:

• same-directory private temporary output and atomic rename;
• refusal to follow or replace an existing symbolic-link output;
• explicit missing-value diagnostics for --input and --output;
• Escape-to-clear filtering, with Playwright coverage;
• updated security and validation documentation.

PR #31 remains the only implementation PR for DEN-1301. No temporary transformation scripts, workflow scaffolding, or competing release-planning behavior from this branch will be merged.

[linear-code]

DEN-1301