feat(sync): selective project export with TUI checkboxes and --projects flag

[alvarorestrepo]

Summary

Adds the ability to choose which projects to export during engram sync, instead of always exporting all or just the CWD project.

What changed

• --projects=a,b flag — non-interactive multi-project export for scripts/CI
• Interactive TUI selector — on TTY, engram sync prompts "Show projects pending sync? [y/n]" and launches a checkbox screen to pick projects
• TUI keyboard bindings — j/k navigate, space toggles, enter confirms, q/esc cancels
• Per-project export loop — each selected project creates its own chunk; results are accumulated and printed as a summary
• --all warning — prints to stderr when --all overrides a --projects value
• Full backward compatibility — --all, --project (singular) and CWD default behave exactly as before; non-TTY environments skip the prompt entirely

Usage

# Interactive (TTY):
engram sync
# → "Show projects pending sync? [y/n]: y"
# → TUI with checkboxes: j/k move, space toggle, enter confirm

# Non-interactive (scripts/CI):
engram sync --projects=engram,gentleman-dots

# Existing flags unchanged:
engram sync --all
engram sync --project myproject

Tests

277 tests passing across all 8 packages. New tests added:

• TestResolveProjects — 8 table-driven cases covering all precedence paths
• TestHandleProjectSelectorKeys* — 7 unit tests for TUI state machine (cursor nav, toggle, enter/cancel, q/esc)

Files changed

File	Change
cmd/engram/main.go	--projects flag, resolveProjects() helper, TTY detection, export loop
internal/tui/model.go	ScreenProjectSelector, 5 new fields, NewProjectSelector() constructor
internal/tui/update.go	Screen router case, handleProjectSelectorKeys(), collectSelected()
internal/tui/view.go	View router case, viewProjectSelector() renderer
cmd/engram/main_test.go	TestResolveProjects table tests
internal/tui/update_test.go	TUI state machine tests