feat(dashboard): decouple Pages sections + single deployer, add pip nav link

[WangLingxun]

Summary

Make the Primus engineering dashboard's Pages publishing decoupled and single-deployer.

Report content now lives on a dedicated orphan dashboard-data branch (weekly/monthly/backend-gap), pushed separately; tooling + site shell stay on main. The deploy workflow overlays the data branch's docs/ before building, so routine report runs never touch main. Sibling static sites (e.g. the DeepSeek-V4 projection) are aggregated as subpaths via a declarative manifest instead of a second, clobbering Pages deployer.

Commits

• feat(dashboard): decouple Pages sections + single deployer, add pip nav link — add tools/backend_gap_report/pages-sections.json (sections = static dirs mounted at /<name>/; links = nav-only links such as the pip index); build_site_bundle.py aggregates them and emits sections.json; the homepage nav is data-driven (probes ./simple/ so the pip link shows only when present). Removes the hardcoded (mis-pathed, silently-skipped) projection copy. Deletes deploy-projection.yml (it competed for the single Pages site) — projection now publishes via the aggregator, and pre-merge preview is done by running the deploy workflow manually on a dev branch. Deploy workflow overlays the dashboard-data branch and drops the daily schedule (a companion workflow on the data branch auto-dispatches on push). Deletes the migrated dashboard data under docs_deprecated/.
• docs(skill): note backend-gap data lives on the dashboard-data plane — trim the backend-gap skill to a one-line data-location note (branch/commit/deploy orchestration is owned by the caller, not the skill).

Notes

• Companion piece (separate branch): the dashboard-data branch is already pushed with the migrated reports; it also carries notify-dashboard-deploy.yml (auto-dispatch on data push). Merge order: merge this PR to main first, so a data-branch push dispatches the new overlay-based deploy.
• Single Pages site tradeoff: a repo has one Pages site, so a dev-branch preview (manual workflow_dispatch) temporarily replaces production until the next normal deploy.
• Wheel publishing unaffected: release-build-wheel.yml is independent; the /simple/ pip index step is preserved.