Add primary plugin tabs and fresh-default worktrees

[dilipgv]

Summary

• add the experimental public primary-tab SDK registration and lifecycle surface
• render persistent host-owned bottom tabs with deterministic ordering/startup, deep-link preservation, route recovery/persistence, semantic badges, responsive layout, and keyboard/focus semantics
• register Tasks as the second primary tab and route it directly to ?view=board
• resolve managed fresh-default worktrees to the refreshed remote default and retain explicit named/local base behavior

Validation

• 130 focused tests across app, plugin SDK, server, host-workspace, and Tasks
• Turbo typecheck for @bb/app, @bb/server, @bb/plugin-sdk, bb-plugin-tasks, @bb/templates, and @bb/domain
• affected-package lint (warning-only repository baseline, no errors) plus clean targeted ESLint for the new primary-tab component/tests
• generated plugin SDK declarations and template copies are current

Compatibility notes

• @bb/plugin-sdk advances additively from 0.4.2 to 0.4.3; the new slot is experimental and documented in docs/api_to_audit.md
• plugin registration sets treat primaryTabs as optional, so existing frontend bundles and plugins that do not register a primary tab continue unchanged
• Tasks raises its declared SDK floor to 0.4.3 because it consumes the new API
• explicit { kind: "named", name: "..." } worktree bases still bypass fresh-default resolution, preserving offline/local-base workflows
• no host-daemon protocol version bump: no daemon wire field or meaning changed
• this tranche adds no Tasks lane model and no Chief plugin implementation

AGENT GENERATED: by GPT-5.6