feat(instantly): Instantly.ai cold email integration (#19)

[jackulau]

Closes #19

Summary

Adds a new instantly integration to @jam-nodes/nodes with three operation nodes and a Bearer-token credential, following the existing apify integration pattern. Implements all five acceptance criteria from the issue with 33 unit tests and no regressions to the existing suite.

What's in this PR:

• instantly_add_lead — add a lead to a cold email campaign (POST /leads)
• instantly_create_campaign — create a campaign with a multi-step drip sequence (POST /campaigns)
• instantly_get_analytics — fetch per-campaign metrics (GET /analytics/campaigns/:id)
• instantlyCredential — Bearer-token credential definition via defineBearerCredential
• instantly?: { apiKey: string } added to NodeCredentials in @jam-nodes/core
• 33 unit tests covering credential metadata, schema validation, happy paths, error paths, body-level success: false, fallback branches, and error-text truncation

Design notes:

• Follows packages/nodes/src/integrations/apify/ as the closest template (Bearer auth, multi-operation, flat test file, shared schemas.ts)
• All three nodes stay pure per CLAUDE.md: no retry/cache/timeout logic in node bodies — that's delegated to fetchWithRetry config ({ maxRetries: 3, backoffMs: 1000, timeoutMs: 30000 })
• No z.any() or as any anywhere
• Error response text is truncated to 500 chars to avoid leaking large API responses into logs
• Size caps on customVariables (max 50 keys), emailAccounts (max 50), and sequence (max 100 steps) to prevent unbounded payloads
• When the API responds HTTP 200 with { success: false, error: ... }, the node returns { success: false, error } instead of silently surfacing the failure as success
• getAnalytics URL-encodes campaignId via encodeURIComponent to defend against path injection
• Exports wired through packages/nodes/src/integrations/index.ts and packages/nodes/src/index.ts following the Slack/Discord pattern

Test results:

• Instantly test file: 33/33 passing (run: cd packages/nodes && npx vitest run src/integrations/instantly/instantly.test.ts)
• Full @jam-nodes/nodes suite: 224/228 (baseline on main was 191/195 — this PR adds +33 passing tests and introduces zero regressions)
• @jam-nodes/core typecheck: clean (cd packages/core && npm run typecheck → exit 0)
• @jam-nodes/nodes typecheck: 324 errors on baseline, 324 errors with this PR — all pre-existing in src/transform/*.ts and src/logic/*.ts, unchanged by this PR

Acceptance Criteria

• Credential definition
• All 3 operations
• Sequence support
• Zod schemas
• Unit tests