Prune orphan prior prompt values on pack removal

[bguidolim]

Summary

When a pack is deselected via mcs sync or removed via mcs pack remove, its declared prompt keys linger in state.resolvedValues indefinitely. Nothing reads them, but a later-installed pack that happens to declare the same key will see the stale value as a "prior" and try to reuse it through the #339 reuse flow.

Why this matters

Cross-pack key reuse is rare but plausible — BRANCH_PREFIX, LABEL_PREFIX, DEFAULT_ASSIGNEE are the realistic collision candidates (conventional keys multiple packs would reasonably reuse). The worst-case symptom after a pack swap:

• User removes pack A (which used BRANCH_PREFIX = "bruno")
• User adds pack B (also declares BRANCH_PREFIX)
• First mcs sync after adding B shows the Reuse N values? gate with B's "prior" of "bruno" — which was actually A's answer the user never intended to share

No data corruption: the user can say n and re-answer. But the gate surfaces a value the current selection never authored, which is surprising.

Proposed fix

In Configurator.unconfigurePack (Sources/mcs/Sync/Configurator.swift), after removing the pack from state:

1. Walk the removed pack's declared prompt keys.
2. For each, check whether any remaining selected pack still declares the same key (via declaredPrompts(context:)).
3. If no remaining pack declares the key, delete it from state.resolvedValues.

declaredPrompts(context:) needs a ProjectConfigContext — the pack being unconfigured is already in memory at that point, and a minimal context can be synthesized.

Scope

• Sources/mcs/Sync/Configurator.swift — unconfigurePack augmentation
• Sources/mcs/Core/ProjectState.swift — new helper like mutating func removeResolvedValues(keys:) (or inline the dict mutation)
• Tests: add a lifecycle case to PromptValueReuseLifecycleTests — pack A with key X → remove A → pack B declaring same X → assert B is asked fresh

Related

• Reuse previously-resolved prompt values on re-sync #339 — introduced the reuse flow and noted pruning as a deferred cleanup
• .claude/memories/decision_architecture_prompt_value_reuse_on_resync.md — documents the "not pruned yet" gap