chore: monorepo workspace scaffolding (#123)

[justn-hyeok]

Summary

• Creates pnpm workspace foundation for monorepo migration
• 6 package stubs with correct dependency graph:

                   shared
                  /  |  \
                core  |   |
               / | \  |   |
          github | notifications
               \ | /
                cli ← tui
  

• Per-package package.json, tsconfig.json, placeholder src/index.ts
• Root workspace scripts (build:ws, test:ws, typecheck:ws, lint:ws)
• Zero impact on existing monolith — all 1506 tests pass

Changes

File	Change
pnpm-workspace.yaml	New: workspace config
tsconfig.base.json	New: shared compiler options
packages/*/package.json	New: 6 package stubs
packages/*/tsconfig.json	New: 6 tsconfig with project references
packages/*/src/index.ts	New: 6 placeholder entry points
package.json	Added workspace scripts

Closes #123

Test Plan

• All 1506 existing tests pass
• Package structure matches design doc
• Dependency graph is correct