feat(appstash): add per-context vars, getClientConfig with 3-tier resolution

[pyramation]

feat(appstash): add per-context vars and getClientConfig with 3-tier resolution

Summary

Extends ConfigStore with two new capabilities for CLI credential bridging and local key-value config:

1. Per-context key-value vars (setVar/getVar/deleteVar/listVars) — stored in ~/.toolname/config/vars/<context>.json, scoped per-context so different environments can have different values (e.g. orgId, defaultDatabase).

2. getClientConfig(targetName, contextName?) — returns { endpoint, headers } using 3-tier resolution following the AWS SDK pattern:

   • Tier 1: appstash store (context endpoint + credentials)
   • Tier 2: env vars (TOOLNAME_TOKEN, TOOLNAME_TARGET_ENDPOINT, TOOLNAME_ENDPOINT)
   • Tier 3: throws with actionable error message

Also fixes a pre-existing bug: the module-level DEFAULT_SETTINGS object was shared by reference across all store instances via readJson. When setCurrentContext mutated the returned settings object, it polluted the default for all subsequent stores. Fixed by deep-cloning fallback values in readJson.

Review & Testing Checklist for Human

• Orphaned vars on context deletion: deleteContext does NOT clean up vars/<name>.json. Verify this is acceptable or if cleanup should be added.
• readJson deep-clone via JSON.parse(JSON.stringify(...)): Works for JSON-serializable fallbacks (which is all current usage), but would silently drop non-serializable values if ever used with them. Confirm this is acceptable.
• getClientConfig returns endpoint without auth when credentials are missing/expired: Tier 1 intentionally returns the endpoint with empty headers if no valid credentials exist. Verify SDK consumers handle unauthenticated responses gracefully.
• Env var naming convention (TOOLNAME_TARGET_ENDPOINT): This convention will need to be matched by the constructive codegen in Phase 2. Verify the naming pattern is what you want before publishing.
• Run npm test in packages/appstash — all 75 tests should pass (31 new).

Notes

• This is Phase 1 of a two-part implementation. Phase 2 (constructive codegen: generated helpers.ts, config commands, BuiltinNames collision handling) will come after this is published.
• ESLint is broken at the repo level (missing eslint.config.js for ESLint v9) — pre-existing, not related to this change.
• Link to Devin session: https://app.devin.ai/sessions/6f16c8a57231488085f04a68be7d599b
• Requested by: @pyramation

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