Show pack add/remove diff in mcs sync interactive picker

[bguidolim]

Problem

Users are misunderstanding the interactive mcs sync picker:

• They don't select packs to install — the picker looks like a read-only list.
• They don't unselect installed packs to remove them — they don't realize the selection is the desired final state, so they assume they need a separate remove action.

The root cause: mcs sync is declarative/convergent (desired state reconciliation), but the UI presents it as a flat selection. The ○/● markers give no cue that pre-checked items represent current state rather than new picks, and there's no preview of the delta before applying.

Proposed solution

1. Delta-aware markers in the picker

Today Sources/mcs/Core/CLIOutput.swift:520 renders two states (● selected / ○ not). Extend to four states that compare the current toggle against the baseline (previously-installed packs, already seeded in Sources/mcs/Sync/Configurator.swift:64):

Marker	Meaning
● (default)	Installed, staying installed
●+ (green)	Will be added
○- (red)	Will be removed
○ (dim)	Not installed, not selected

Plus a live footer updating on each toggle: +2 to add, -1 to remove, 3 unchanged.

2. Confirmation screen before apply

After Enter, render a summary and require y to apply:

Review changes (project: /path/to/repo)
  + add:     ios, github-planner
  - remove:  android
  = keep:    swift

Apply? [y/N]

This catches the misunderstanding even if someone misreads the picker. Skippable when there's nothing to remove (additions-only is the friendly case).

3. Reframe the section header

Rename the picker title from "Tech Packs" to something like "Desired packs (selected = installed after sync)" in Configurator.swift:68-73. One line of copy that reframes the whole interaction.

Tradeoff considered and rejected

Tempting alternative: add mcs install <pack> / mcs remove <pack> as verbs matching user intuition. Rejected because it fragments the convergence engine — two mental models for the same operation, and ProjectState / ProjectIndex ownership tracking assumes a single reconciliation path. Keep sync as the single source of truth and make its intent clearer.

Scope

Touches CLIOutput.swift (picker rendering, new SelectableItem.baselineSelected field) and Configurator.swift (baseline propagation + post-picker confirmation step). Also applies to --global sync since it uses the same picker.