feat(test): add integration test layer (no credentials required)

[fredaluliu]

Background

Current E2E tests all require real Tencent Cloud credentials and live sandbox instances, making PR feedback slow and blocking external contributors who don't have credentials.

Proposal

Add a new tests/integ/ layer that validates CLI behavior without any cloud credentials:

• Compile the real agr binary
• Run commands in a subprocess with credentials cleared
• Assert on: JSON Envelope structure, help text, exit codes, error classification, schema output

This mirrors the pattern already used in cmd/agr/characterization_test.go but as a dedicated CI job.

Scope

• agr --help / agr <cmd> --help output validation
• agr schema <cmd> -o json structure validation
• agr explain <code> output validation
• agr version -o json structure validation
• agr doctor (no-credential mode) output validation
• Error exit codes for invalid inputs (exit 2)
• JSON Envelope schema compliance (agr.v1)

Expected Outcome

• New tests/integ/ directory with test files
• New CI job that runs in <30s without credentials
• External contributors can validate their changes pass CI without needing cloud access

Category

Testing / DX

[fredaluliu]

TAPD: https://tapd.woa.com/tapd_fe/70204071/story/detail/1070204071136154116