feat(sync): selective project export with TUI checkboxes

[alvarorestrepo]

Summary

This PR implements Selective Project Sync, allowing users to choose which projects to export instead of relying solely on CWD auto-detection.

Changes

CLI (cmd/engram/main.go)

• Add --projects=a,b flag for non-interactive multi-project export (comma-separated)
• Add interactive y/n prompt on TTY: "Show projects pending sync? [y/n]"
• Implement resolveProjects() helper with strict precedence: --all > --projects > --project > TUI > CWD default
• Replace single syncExport() call with per-project export loop
• Warn to stderr when --all overrides --projects
• Preserve 100% backward compatibility with existing --project, --all, and CWD default behavior

TUI (internal/tui/)

• model.go: Add ScreenProjectSelector, NewProjectSelector() constructor, 3 state fields (ProjectSelectorItems, ProjectSelectorChecked, ProjectSelectorCursor)
• update.go: Add handleProjectSelectorKeys() with j/k navigation, space toggle, enter confirm, q/esc cancel. Uses buffered channel for result handoff.
• view.go: Add viewProjectSelector() for checkbox list rendering

Testing

• Add TestResolveProjects() – table-driven with 8 cases (flag parsing, precedence, TTY fallback, CWD default)
• Add TestHandleProjectSelectorKeys*() – 7 test functions covering all navigation and toggle states
• 277 tests passing, 0 regressions across all 8 packages

Usage Examples

# Interactive (TTY) – prompts for project selection
engram sync

# Non-interactive – specific projects
engram sync --projects=engram,dots,work

# Backward compatible – single project
engram sync --project=legacy

# All projects (no prompt)
engram sync --all

# Piped/CI – skips prompt, uses CWD default
engram sync | tee sync.log

Issue

Closes #107