Prompts should be skippable when their components aren't selected

[bguidolim]

Problem

PromptDefinition (Sources/mcs/TechPack/PromptDefinition.swift) has no condition field, and CrossPackPromptResolver.collectDeclaredPrompts (Sources/mcs/Sync/CrossPackPromptResolver.swift:20-25) gathers prompts per pack, not per selected component:

packs.flatMap { $0.declaredPrompts(context: context) }

So a prompt is asked even when every component that consumes its placeholder was deselected via mcs sync --customize, and the answer goes nowhere. Harmless with one or two packs, increasingly irritating as packs accumulate prompts.

Sketch

Add an optional requiredBy: [String] (component ids) to PromptDefinition:

prompts:
  - key: KB_GATE_MODE
    type: select
    requiredBy: [hook-gate-turn, hook-gate-record, hook-gate-check]

Skip the prompt when none of the listed ids are in the selected set; keep asking when requiredBy is absent (current behavior).

Insertion point: ProjectConfigContext is already threaded into declaredPrompts(context:) and is currently under-used — implementations read only isGlobalScope from it. Carrying the selected-component set on the context is cheaper than changing the collectDeclaredPrompts signature. The selection is known at Configurator.resolveAllValues (Configurator.swift:645).

Two details to settle before starting

1. Placeholder fallback is bigger than it looks. ConfiguratorSupport.scanForUndeclaredPlaceholders (called at Configurator.swift:695) takes packs, not the selected component set — so it scans deselected components' files too. A requiredBy-skipped prompt would then be re-asked as a raw Set value for KB_GATE_MODE inline prompt even when nothing selected uses it, which is strictly worse than today's behavior.

Narrowing that scan to selected components is therefore part of this work, not a follow-up — and it needs care: the scan is load-bearing as the safety net for emitWarnings: false substitution, and covers three artifact families (copyPackFile sources, settingsMerge file sources, mcpServer env/command/args).

Alternative worth considering: treat a requiredBy mismatch — a placeholder used by a component not listed in requiredBy — as an authoring error at mcs pack validate time. It's statically detectable and avoids the runtime question entirely.

2. Prior values. A skipped prompt must leave any stored prior untouched rather than clearing it, so re-enabling the component later doesn't lose the answer.

Validation to add

Every id in requiredBy must exist in the pack's components — catches typos and renames at mcs pack validate time.

Tests

Tests/MCSTests/CrossPackPromptResolverTests.swift:

• prompt with requiredBy and no listed component selected → not asked
• prompt with requiredBy and one listed component selected → asked
• prompt without requiredBy → asked (regression guard)
• skipped prompt leaves prior value intact across a re-sync
• requiredBy referencing an unknown component id → validation error

[bguidolim]

Re-checked this against current main. Every claim still holds:

• CrossPackPromptResolver.collectDeclaredPrompts is still packs.flatMap { $0.declaredPrompts(context: context) } (CrossPackPromptResolver.swift:24)
• Configurator.resolveAllValues (Configurator.swift:642) receives packs but not excludedComponents, even though configure() holds it at :228 and threads it into five other steps
• scanForUndeclaredPlaceholders (ConfiguratorSupport.swift:358-412) walks pack.components unfiltered, across exactly the three artifact families noted
• Prompts are manifest top-level (ExternalPackManifest.swift:16) with no component linkage anywhere — requiredBy would be the first component↔prompt edge in the model

Splitting out the two parts that are bugs today, independent of requiredBy:

• Plain interactive 'mcs sync' wipes component exclusions chosen via --customize #363 — plain interactive mcs sync wipes --customize exclusions and erases them from state (Configurator.swift:128). --pack/--all honor persisted exclusions; the interactive non-customize path does not.
• Undeclared-placeholder scan ignores --customize exclusions, prompting for deselected components #364 — the undeclared-placeholder scan prompts for placeholders that appear only in deselected components. This is "Detail 1" from the description above, promoted to its own issue: it misfires today, with no requiredBy involved, and it is a prerequisite for this one.

Two corrections to the sketch above, for whenever this is picked up:

The validate-time alternative isn't an alternative. Static checking can't stop the scan from finding __KB_GATE_MODE__ inside a deselected component's files, so narrowing the scan is unavoidable if requiredBy ships. The two are complementary: narrowing handles deselected components, validation catches the inverse (a selected component using a placeholder whose prompt's requiredBy omits it), where the runtime fallback would otherwise degrade to a raw Set value for X.

Narrow at the call site, not inside declaredPrompts. unconfigurePack calls pruneOrphanResolvedValues, which drops state.resolvedValues keys not declared by surviving packs. If declaredPrompts starts returning a selection-narrowed list, that pruner would eat the stored answers for skipped prompts — exactly the prior-value loss "Detail 2" is trying to prevent. Filter in collectDeclaredPrompts / groupSharedPrompts instead, and leave TechPack.declaredPrompts returning the full pack-level list so the pruner and doctor keep their complete view.

Status: keeping this open, but relabeling to idea / P5.

With #363 and #364 fixed, the residue here is real but narrow: a pack that declares a prompt for a component you deselected still asks for it. That's a genuine ergonomic gap, not a correctness one — the answer is stored harmlessly and reused if the component is ever re-enabled. Against that: it's a permanent addition to the manifest schema, nobody has hit it in practice yet, and its design depends on exclusion semantics that #363 is about to change. Recording the design rather than queueing the work is the right trade until a pack actually accumulates enough prompts to make it bite.