chore: remove the q_dev plugin

[warren830]

Stacked delivery

Depends on #9078. Merge only after the full chain #9075 -> #9076 -> #9077 -> #9078.

This is intentionally the final layer: q_dev remains available until every backend, Config UI, E2E, and dashboard consumer has migrated. This PR is ready for review, but do not merge it until its direct dependency lands.

After the preceding PRs merge, the clean layer is 68 files with 7,190 deletions and 3 insertions.

Summary

• remove the q_dev backend plugin, migrations, tests, and obsolete documentation
• remove the q_dev Config UI registration and fields
• remove the q_dev full-flow E2E after the Kiro replacement is present
• keep the all-Go-plugin schema guard synchronized with the final plugin set
• update the remaining GitHub Copilot documentation reference

Validation

• go test ./plugins/kiro/... -count=1
• go vet ./plugins/kiro/...
• go build ./plugins/kiro/...
• Config UI production build
• Playwright discovery: 13 Kiro tests
• plugin registration guard: 44 impl directories / 44 registered plugins
• q_dev runtime references: zero

Signed-off-by: warren warren.chen830@gmail.com