feat: own M6 learning shortcuts

[yuezengwu]

What changed

• register Explain-owned selection and finalized-answer shortcuts on first-party DSH slots
• create editable /explain --selection and exact-turn /explain --answer drafts without automatic submission
• keep optional selection-chat, suggested-replies, and Advisor plugins unmodified and independent
• preserve legacy --suggested input and persisted origin: suggested history as read compatibility
• update the M6 product, architecture, acceptance, and release documentation

Why

M6 previously placed integration changes in consumer plugins. Keeping both adapters inside Explain gives the feature one owner, removes cross-repository release coupling, and still allows users to select visible content produced by optional plugins.

Validation

• pnpm run typecheck
• pnpm test (64 tests)
• DSH_SOURCE_DIR=/tmp/dsh-official-latest.CUnRHD/source pnpm run test:web (4 tests)
• DSH_SOURCE_DIR=/tmp/dsh-official-latest.CUnRHD/source pnpm run test:m6 (3 tests)
• DSH_SOURCE_DIR=/tmp/dsh-official-latest.CUnRHD/source pnpm run dsh:link:check
• npm pack --dry-run --json
• git diff --check

Real-model GUI evidence

• Explain commit: c64a76224ba77ef650be8cad9572281e7f57f4ba
• DSH commit: 47f943859bef60e4160492346772ded9b24f765a
• Server: built production Web profile from those exact clean trees, with Explain installed from the recorded checkout
• State: fresh DSH_HOME, DSH_AGENTS_HOME, workspace, real Session, and isolated browser context
• Models: one real DeepSeek main-agent round created the source answer; one real deepseek-chat Explain round created the learning card
• Recording: repository-declared Playwright in isolated headless Chromium; this was the documented fallback because the in-app browser could not drive a real text drag reliably
• No fixture query, mock transport, synthetic selection, or test-only hook was used
• GIF SHA-256: 6c20446187f3ca09a701d28e50a81c4b285a0aeafa2f180a61ce63201d874f33