bridge: add todo.sr.ht (SourceHut) bridge - #1499

Record the API schema beside the bridge so client queries, mutations, and response types can be reviewed against the service contract without relying on an external copy.

Add authenticated HTTP transport, GraphQL request and response types, pagination queries, and ticket, event, and label mutations. Keep the protocol client separate from bridge synchronization and configuration.

Register the bridge, resolve tracker credentials, import remote tickets and events, and export local operations. Combine the mutually dependent importer and exporter so the bridge can compile as one implementation commit.

Provide mock GraphQL methods and tests for the client, configuration, debug transport, importer, and exporter. Fold compilation repairs into the tests rather than leaving later fixes to earlier test commits.

Document tracker URL, login, token, and bridge usage in help and reference pages. Include the design plan with the user-facing guide.

… handling

Address feedback from code review on the todo.sr.ht bridge:

- Require verified tracker and base URL metadata before accepting
  creation metadata ticket IDs on export, preventing operations on
  unverified bugs from modifying tickets on the configured tracker.
- Track imported event operations with change-level IDs to prevent
  ErrMultipleMatch on repeat pulls of events containing multiple changes.
- Reconcile remote edits to ticket title and body during import when
  reusing existing bugs.
- Support explicit null body in UpdateTicketInput to allow clearing
  ticket descriptions on SourceHut without losing edits.
- Make label export idempotent by checking remote ticket labels before
  adding or removing them.
- Ensure credentials added without base URL metadata can be used to
  initialize the bridge, and tag selected credentials with base URL.
- Isolate user identities by instance base URL to avoid collisions
  across multiple SourceHut instances.
- Align ValidParams and error messages with the --project flag.
- Establish chronological order for imported events across pages.
- Silence repetitive warnings for unsupported assignment and mention events.
- Surface server GraphQL error details on non-200 HTTP responses and clean
  unused variables when querying the authenticated user.
- Document label color limitations and replace the obsolete roadmap
  in the design documentation.

Scope ticket queries, tracker mappings and credentials to their owner and instance. Preserve pending local edits, record remote content checkpoints, and stop import/export from replaying state changes out of order. Keep ambiguous comments separate and avoid attributing unknown editors to ticket submitters. Add regression coverage and regenerate the add-token documentation.

Compare checkpoints, remote content and local snapshots in the same cleaned form, including legacy baselines. Skip unsupported ordinary-comment edits before credential lookup, while preserving the ordered stop for supported changes. Add regressions for false conflicts, continued event import and credential-free skips.

Pass the command context through configuration, identity and bug
resolution, commits, export metadata, and remote-state
checkpoints. Configuration now uses the same context for
authenticated requests instead of a placeholder context, so
cancellation reaches those requests.

Update regression tests for the context-aware cache API. Commit
created bugs before resolving or exporting them, and persist the
initial imported issue before testing a later remote edit. Trunk
no longer commits bug creation implicitly, so these fixtures
otherwise test empty repositories rather than synchronization.

Refs: git-bug#1499