Feature: Add support for DeepSeek Harness (dsh) as an install target

[pinghe]

Describe your idea
Please add DeepSeek Harness (dsh) as a supported install target in the BMad installer, so that npx bmad-method install can deploy BMad skills into a DSH workspace alongside the currently supported tools (Claude Code, Cursor, Codex, etc.).

Why is this needed?
DeepSeek Harness is an open-source agent harness developed by DeepSeek AI (installable via npx @deepseek-ai/dsh web). It follows the same cross-tool skill standard that BMad's config-driven installer already targets: it discovers Agent Skills from .agents/skills (project level) and ~/.agents/skills (user level, DSH_AGENTS_HOME-overridable), reading SKILL.md files with name / description / whenToUse frontmatter.

Supporting it would:

• Let the growing DSH user base run BMad workflows with zero manual copying of skills.
• Cost the project almost nothing: DSH reads the shared .agents/skills directories that several existing platform entries already use, so no bespoke adapter should be needed.
• DSH hot-watches its skill roots, so skills installed by the BMad installer appear immediately without a restart.

How should it work?
Add a dsh entry to tools/installer/ide/platform-codes.yaml:

dsh:
  name: "DeepSeek Harness (dsh)"
  preferred: false
  installer:
    target_dir: .agents/skills
    global_target_dir: ~/.agents/skills

That should be all that's required, since ConfigDrivenIdeSetup handles shared .agents/skills targets generically. Implementation notes:

• Skill format is already compatible: DSH parses SKILL.md frontmatter (name, description, whenToUse) and resolves scripts/assets relative to the skill directory, which matches how BMad packages skills.
• Verification steps: run the installer with dsh selected in a project, launch dsh web, confirm BMad skills show up in the skill catalog and are invokable; edit a skill file and confirm hot-reload picks up the change.
• Optionally mention DSH in tools/docs/native-skills-migration-checklist.md so the docs stay consistent with the new entry.

Additional context

• DSH repository: https://github.com/deepseek-ai/deepseek-harness
• Documentation: https://deepseek-harness.github.io/deepseek-harness/
• Skill discovery roots (from packages/skill/skill-filesystem): <project>/.dsh/skills, <project>/.agents/skills, ~/.dsh/skills, ~/.agents/skills — the shared .agents standard is the right target for BMad.
• Note: DSH is currently in developer preview and iterating quickly, so the entry is best added with the shared cross-tool paths rather than DSH-private paths.