feat(appstash): add multi-target context support

[pyramation]

feat(appstash): add multi-target context support

Summary

Extends ContextConfig with an optional targets map so a single context can store per-API endpoint overrides (e.g., auth, members, app each pointing to different GraphQL endpoints). Adds getTargetEndpoint(targetName, contextName?) to resolve the correct endpoint for a given target, falling back to the context's main endpoint when no target-specific override exists.

This is a prerequisite for unified multi-target CLI generation in @constructive-io/graphql-codegen, where a single codegen config can define multiple API endpoints that share one set of credentials.

New types:

• ContextTargetEndpoint — { endpoint: string } (extensible for future per-target config)

Changed types:

• ContextConfig.targets? — optional Record<string, ContextTargetEndpoint>
• ConfigStore.createContext() — accepts optional targets in options
• ConfigStore.getTargetEndpoint() — new method

Fully backward compatible — existing single-endpoint contexts work identically.

Review & Testing Checklist for Human

• Fallback behavior for getTargetEndpoint: When a target name is not found in ctx.targets, it silently falls back to ctx.endpoint rather than returning null. Verify this is the desired behavior — a misspelled target name in a multi-target context would silently route to the wrong endpoint. Consider whether this should be configurable or at least log a warning.
• Run pnpm test in packages/appstash to confirm all 54 tests pass (46 existing + 8 new).

Notes

• Link to Devin run: https://app.devin.ai/sessions/e9c668834d394cdc8ebcb371b6ebf430
• Requested by: @pyramation

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