Split Plugins and Skills into separate workspaces

[brsbl]

Human comments

What was wrong

• Plugins and Skills shared an Extensions entry, workspace sidebar, and top navigation instead of having separate destinations.
• Plugin and Automation icons differed between the sidebar and + menu.

What changed

• Replace Extensions with sibling Plugins and Skills entries, each with its own workspace navigation: Browse plugins / Installed plugins and Browse skills / My skills.
• Make those four subpage rows text-only; preserve their labels, routes, and active states.
• Use /plugins and /skills as canonical routes. Redirect legacy /extensions/... and /tools/... paths while preserving path parameters, query strings, and hashes.
• Redirect /settings/plugins to /plugins?view=installed. Keep individual /settings/plugins/:pluginId configuration pages unchanged.
• Preserve existing page functionality, shipped SDK identifiers, and saved sidebar order/visibility preferences.
• Match icons across the main sidebar and + menu: Plug02 for Plugins, the original Zap for Skills, and Repeat for Automations. No hover icon swapping.

How you verified

• Remote CI passed on f5a77a2ce: app, server, integration, package, type/lint, and Linux/macOS packaging checks. All 11 resource-sidebar tests pass, including icon-free rows, destination links, workspace ownership, and active states. Existing redirect and sidebar-preference coverage also passes.
• Chrome for Testing 151.0.7922.71, isolated branch web app on f5a77a2ce: exercised all four subpage links, active highlighting, return navigation, My skills after reload, and sidebar/menu transitions. Confirmed text-only subpage rows and unchanged main sidebar/+ menu icons. No prompts were submitted or interaction errors observed.
• Verified light-theme desktop web at 1440×900 and phone-responsive web at 390×844, both DPR 1. Phone screenshots show the open sidebar; this is responsive web coverage, not native iOS verification.
• Before screenshots use exact merge base 647337060; all after screenshots use exact PR head f5a77a2ce. Matched fixtures, corresponding legacy/canonical pages, light theme, and software rendering. Main navigation uses an empty project/thread fixture and Codex 5.6-Sol Medium; desktop captures show the open + menu, phone capture shows the open sidebar. Plugins captures use the Kanban example.

View	Before — 1440×900	After — web/desktop 1440×900	After — mobile 390×844
Main navigation
Browse plugins
My skills

BB-Thread-ID: thr_czdxmmyenv

AGENT GENERATED

[brsbl]

Superseded by #3384, a standalone, single-commit PR against main. The replacement preserves the same feature patch (stable patch ID verified), passes remote CI, and includes refreshed before/after desktop and phone screenshots. Closing this PR preserves its discussion and stack history; nothing is being merged into main.