feat(bridge): add Azure DevOps (Azure Boards) bridge - #1590

Summary

Bidirectional bridge between git-bug and Azure DevOps Boards work items, requested in #1542. Authentication is via Personal Access Token (PAT). The REST client is hand-rolled on net/http with no new dependencies.

Per @sudoforge's note on #1542: I will maintain this in-tree until the external plugin interface lands, and I take responsibility for issues related to it.

Opening as a draft for early feedback on the three scoping questions I raised on #1542 (target name, client approach, single vs. split PR).

Changes

New bridge/ado/ package, registered in bridge/bridges.go:

• ado.go: BridgeImpl, metadata/config constants, PAT client builder
• client.go: REST client for WIQL, get/create/update/delete work items, updates (revisions), and comments
• config.go: Configure() PAT flow (interactive and non-interactive)
• import.go: incremental WIQL on System.ChangedDate, mapping work items, comments, state, tags and description edits to bug operations, with a final-status reconcile for items created directly in a closed state
• export.go: create and update work items via JSON Patch, replaying comments, status, title and label changes
• mapping.go: pure helpers (WIQL builder, state/tag mapping, identity parsing)

One bridge per Azure DevOps project, matching the Jira bridge model. Config keys beyond base-url/organization/project/login are optional: create-work-item-type (default Task), closed-states, open-state-target, closed-state-target.

Test plan

• go test ./bridge/ado/: 23 unit tests (mapping logic + net/http client via httptest)
• go vet and gofmt clean; go.mod / go.sum unchanged
• Live-tested end to end against a real Azure DevOps organization:
  • Import: auth, WIQL incremental query, field/identity/tag/comment/revision parsing, importer to git-bug bugs, closed-item replay, and idempotency (a second pass is a no-op)
  • Export: create work item, comment, close and label, then verified on the remote
• Live integration tests are gated behind ADO_TEST (read-only) and ADO_TEST_EXPORT (writes), matching the github/gitlab bridge convention, so they skip in CI.

Known follow-ups

• PAT-only auth for now; OAuth can follow.
• Project-scoped; area-path or team scoping could come later.
• A doc/azuredevops_bridge.md page once the scoping questions are settled.