[Feature] Introduce CI integration tests with provider health gates

[mbe24]

Problem

External integration tests currently mix very stable fixtures and volatile dependencies in one lane. This creates noisy failures from missing/expired credentials and endpoint drift, while also making it harder to tell infrastructure issues from adapter regressions.

Design

Introduce two external-dependency CI tiers plus workflow-level provider health gates:

1. Tier A (blocking): stable external fixtures.
2. Tier B (advisory): token and instance dependent providers.
3. Per-provider smoke checks (env + auth/reachability) run before full provider test slices.

Initial mapping:

1. Tier A: GitHub, GitLab.
2. Tier B: Jira, Bitbucket Cloud.

Scope

• Add dedicated external integration workflow with per-provider jobs.
• Add workflow-level health/auth gates before full provider test slices.
• Keep Tier A blocking and Tier B advisory.
• Execute provider-specific integration slices by name prefix filters.
• Document fixture and credential requirements for CI and local runs.

Boundary

• No adapter behavior changes in this issue.
• No migration of integration tests into multiple files in this phase.
• No immediate promotion of Jira/Bitbucket to blocking.

Acceptance Criteria

• Tier A provider jobs block PRs and fail clearly on broken fixtures/auth.
• Tier B provider jobs run as advisory and do not block merges.
• Each provider job reports whether failure was due to missing env, invalid token, endpoint health, or test assertions.
• External workflow can be run manually and on schedule with predictable behavior.
• Core blocking CI jobs remain unchanged and stable.

Context

The project now has broader provider coverage and more external integration tests. A tiered strategy improves reliability and triage quality while preserving meaningful blocking checks for stable providers.

• GitHub Actions workflow syntax: https://docs.github.com/en/actions/writing-workflows/workflow-syntax-for-github-actions
• GitHub Actions expressions/conditions: https://docs.github.com/en/actions/learn-github-actions/expressions
• GitHub REST auth and rate limits: https://docs.github.com/en/rest
• GitLab REST API docs: https://docs.gitlab.com/ee/api/
• Jira Cloud REST API docs: https://developer.atlassian.com/cloud/jira/platform/rest/v3/intro/
• Bitbucket Cloud REST API docs: https://developer.atlassian.com/cloud/bitbucket/rest/