Extract PrethinkContextRecipe base class for cycle-2-deferred recipes

[kmccarp]

Background / problem

When the build-time LST manifest includes existing .moderne/context/* files (the post-moderne-cli#3661 reality), the recipes in this module that defer real work to cycle 2 stop firing for repos that already have those files committed. Customer-facing symptom: running UpdatePrethinkContextNoAiStarter across a multi-repo corpus generates context for some repos and silently skips others.

The mechanism: ExportContext.getVisitor returns the tree unchanged on cycle 1 because data tables are populated by other recipes' visitors during cycle 1's edit phase, after this recipe's generate() has already run. ExportContext.generate also defers to cycle 2 for the same reason. The recipe contributes zero changes in cycle 1, and the only thing historically keeping the cycle alive was GenerateCalmArchitecture's placeholder hack (writing a {} file at .moderne/context/calm-architecture.json purely to trick the scheduler into running cycle 2). Once .moderne/context/calm-architecture.json is in the LST, the placeholder check fails, the placeholder isn't written, and RecipeScheduler.runRecipeCycles terminates the loop after cycle 1. GenerateCalmArchitecture has the same fragility, just expressed differently.

The minimal fix for that specific bug is in #16: each affected recipe sets a marker on the ExecutionContext from cycle 1 to keep the scheduler loop alive. This PR is an alternative that fixes the same bug and extracts the shared shape into a base class so future recipes don't have to remember to wire it up.

Solution

Introduce PrethinkContextRecipe, a public abstract ScanningRecipe that absorbs the shared structure of recipes which write to data tables in cycle 1's edit phase and need to read those data tables in cycle 2 to export context files. The base class:

• Provides a fixed Set<Path> existingContextPaths accumulator and a scanner that populates it.
• Marks causesAnotherCycle(), getInitialValue(), getScanner(), and getVisitor() final.
• On cycle 1, the visitor calls ctx.putMessage(Prethink.CYCLE_TRIGGER, true). RecipeRunCycle treats any ExecutionContext mutation as the recipe contributing a change for cycle-2 enrollment. The io.moderne. prefix is required by CursorValidatingExecutionContextView's allow-list.
• On cycle 2, the visitor delegates to a new cycle2Visit(Tree, Accumulator, ExecutionContext) template method. On cycle 3+ it returns the tree unchanged so implementations don't have to be idempotent across cycles.

ExportContext and GenerateCalmArchitecture migrate to extend PrethinkContextRecipe. Their old getVisitor bodies move into cycle2Visit. GenerateCalmArchitecture's placeholder hack and its visitor's cycle != 2 guard become dead code and are removed.

Relationship to #16

This PR and #16 fix the same bug. #16 does it with the smallest possible change (per-recipe ctx.putMessage calls); this PR does it by introducing the base class and migrating the recipes to extend it. Either can ship on its own. If reviewers prefer the minimal fix, merge #16 and close this. If reviewers prefer the framework refactor, merge this and close #16.

Test plan

• Existing tests pass.
• New requestsAnotherCycleWhenNoOtherRecipeMakesChangesInCycle1 regression test in ExportContextTest, and updatesExistingCalmFileEvenWhenNoOtherRecipeMakesChangesInCycle1 in GenerateCalmArchitectureTest. Both invoke Recipe.run directly with maxCycles=3, minCycles=1 -- matching the production CLI scheduler call -- because RewriteTest.rewriteRun cannot represent this scenario through expectedCyclesThatMakeChanges without contradicting the success criterion. Both tests fail without the base class's cycle-1 trigger.
• GenerateCalmArchitectureTest cycle counts updated to reflect the placeholder removal: generatesNothingWithNoDataTables now expects 0 cycles with changes (previously 2: placeholder created, placeholder deleted), and the data-table-populated tests expect 1 cycle (the file gets created in cycle 2 only).
• End-to-end on the production Moderne CLI 4.2.0 against a 3-repo corpus reproducing the customer-reported bug: per-repo .moderne/context/* entries in fix.patch go from 21 + 0 + 0 (broken) to 21 + 16 + 18 (each repo now produces output), byte-identical end state to the pre-#3661 baseline.

What's NOT in scope

• Closed-source recipes that share this shape (GenerateCalmMermaidDiagram in moderneinc/rewrite-prethink) will migrate in a separate PR there once this lands and a release is cut.
• FindCalmRelationships (also closed-source) has the same causesAnotherCycle()=true-without-a-trigger latent bug, but uses a different accumulator shape (Set<String> of in-repo classes, not paths). Stays outside this hierarchy.
• Generalizing PrethinkContextRecipe to support custom accumulator shapes. Deferred until a recipe actually needs it.