feat: Claude Plugin Export for Cowork

[falfaddaghi]

Summary

This PR changes the existing Claude export target into a Claude/Cowork plugin package instead of writing legacy .claude artifacts. The generated package now includes a valid plugin manifest, a run skill, bundled agents/skills/scripts/docs, round-trip Nexus workflow JSON, and plugin-focused README guidance. OpenCode and PI export behavior is preserved while the export dialog and docs now describe the Claude/Cowork plugin folder and ZIP flows.

Plan: docs/tasks/claude-plugin-export-for-cowork-ba5f56d5/plan-claude-plugin-export-for-cowork-ba5f56d5.md

ADW tracking ID: ba5f56d5

Checklist

• Added Claude plugin helper module and regression coverage for plugin names, manifests, resource references, package layout, and validation.
• Refactored generation target path/reference helpers so Claude artifacts are plugin-root-relative while OpenCode and PI keep their existing roots.
• Updated workflow generation to emit .claude-plugin/plugin.json, skills/run/SKILL.md, bundled resources, nexus/<workflow>.json, and plugin README files.
• Removed generated run scripts from Claude plugin exports while preserving OpenCode/PI run script behavior.
• Updated node generators and detail sections to reference bundled Claude resources through ${CLAUDE_PLUGIN_ROOT}.
• Updated browser folder and ZIP export handling for Claude plugin packages.
• Refreshed export UI copy, README guidance, changelog notes, and the E2E test specification.
• Expanded regression tests for generation targets, run scripts, export planning, parallel-agent references, and handoff generation.

Key Changes

• Introduces src/lib/claude-plugin-export.ts for plugin name sanitization, manifest/README generation, skill frontmatter conversion, plugin-root detection, and resource references.
• Generates Claude plugin packages with .claude-plugin/plugin.json at the package root, workflow instructions in skills/run/SKILL.md, bundled agents/skills/docs, and a Nexus workflow JSON snapshot.
• Routes Claude direct folder export into nexus-<workflow-slug> unless the selected folder is already the plugin root; ZIP exports place plugin files directly at the archive root.
• Updates docs and UI language to describe Claude/Cowork plugin packages, including namespaced /nexus-<workflow>:run invocation.