fix(create): exit 5 on InitTask insufficient_credits (0.5.1)

[voidkey]

Summary

A 0.5.1 patch that fixes an exit-code inconsistency surfaced during 0.5.0 real-backend validation.

Before: vk create exited 1 when POST /v1/tasks/init returned insufficient_credits (envelope code 100001), via cobra's generic error handler.
After: exits 5 (business failure), matching:

• the same condition surfacing later in the SSE stream path (cmd/create.go:259 — already os.Exit(5))
• the documented exit-code contract in AGENTS.md / README (5 = business failure)

Agents branching on exit codes no longer need to know about two different paths for the same condition.

Diff

• cmd/create.go: replace return fmt.Errorf(...) with fmt.Fprintln(os.Stderr, ...); os.Exit(5) — verbatim mirror of the stream-side handler 4 lines below.
• tests/integration/create_credits_test.go (new): mocks /v1/tasks/init returning envelope code 100001, asserts binary exits 5 and stderr carries the message.
• CHANGELOG.md: 0.5.1 entry under Fixed.
• package.json + 3 SKILL.md: 0.5.0 → 0.5.1.

Test plan

• go test ./... passes including new integration test
• go build ./... clean
• CI three-platform green (will appear after push)
• Manual smoke against real backend with depleted credits — not retested (already established symptom during 0.5.0 validation; mock test is equivalent shape)

Out of scope

• auth status showing valid but token refresh actually failing — was also observed during 0.5.0 validation but the root cause is non-obvious; needs middleware-level diagnostics before fix. Tracked separately.