feat(ui): add component subpath exports

[kattsushi]

Part of tracker #2. Targets feat/ui-package.

Closes #1

Approved issue: #1

PR Type

• Bug fix
• New feature
• Documentation only
• Code refactoring
• Maintenance/tooling
• Breaking change

Summary

• Adds explicit ergonomic package subpath exports for every public UI component.
• Extends import smoke coverage so each component subpath resolves.
• Keeps chart opt-in via @os-copilot/ui/chart; chart remains absent from the root barrel.

Chain Context

master
└── 📍 tracker #2: feat/ui-package
    ├── ✅ PR #3: scaffold package + shadcn routing
    ├── ✅ PR #4: Zaidan baseline + Storybook/CSS
    ├── ✅ PR #5: layout primitives + SSR/SPA verification
    └── 🔎 PR #6: component ergonomic subpath exports

Changes

File	Change
packages/ui/package.json	Adds explicit ./component-name exports for every public UI component.
packages/ui/src/testing/tree-shaking-smoke.test.ts	Resolves every public component ergonomic subpath.

Test Plan

• pnpm exec dprint check packages/ui/package.json packages/ui/src/testing/tree-shaking-smoke.test.ts
• pnpm exec oxlint packages/ui/src/testing/tree-shaking-smoke.test.ts
• pnpm nx test ui --run
• pnpm nx typecheck ui
• pnpm nx build ui
• Fresh reviewer audit completed with no blockers.

Notes

• This is a small contract PR to close the PRD requirement for per-component subpath exports before adding larger custom components/dependency decisions.

Contributor Checklist

• Linked tracker PR feat(ui): track UI package chain #2
• Added exactly one type:* label
• Ran shellcheck on modified scripts (not applicable: no shell scripts changed)
• Skills tested in at least one agent
• Docs updated if behavior changed
• Conventional commit format
• No Co-Authored-By trailers