fix(dsh): backport PR 579 plugin activation onto v1.2.1

[syzsunshine219]

Summary

• Cherry-pick the DeepSeek Harness plugin changes from merged PR fix(dsh): restore Memmy plugin activation across supported DSH tags #579 that are not already on v1.2.1.
• The recall message source and package.json version are already on v1.2.1. This PR brings the remaining two commits: standalone adapter lifecycle alignment, and removal of the nonexistent @deepseek-ai/dsh-client-runtime client manifest entry.
• Covers the generated plugin templates, the standalone DSH adapter, and their tests.

Test plan

• Confirm the diff against v1.2.1 touches only the five DeepSeek Harness files
• Run the DeepSeek Harness skill target tests and the Memory template tests
• Confirm no commits from main outside PR fix(dsh): restore Memmy plugin activation across supported DSH tags #579 are brought into v1.2.1