Fixes 'concurrent map read and map write' error in TestImporter by
wrapping memfs with a RWMutex for synchronized access.

- Add complete todosrht bridge with GraphQL client and API
  integration
- Implement full CRUD operations for tickets, comments, labels,
  and events
- Add comprehensive test suite with mock client and
  authentication
- Update bridge registry to include TodoSourceHut implementation
- Add configuration management and validation for SourceHut
  instances
- Include import/export functionality for bidirectional sync
- Update documentation with man pages and usage guides
- Add proper error handling and pagination support

Fixes: git-bug#1024
Signed-off-by: Matěj Cepl <mcepl@cepl.eu>

Add support for full URL parsing and enhance error reporting for
better user experience:

* Add parseTodoURL() to extract base URL and tracker from full
  URLs like https://todo.sr.ht/~owner/tracker
* Implement two-step validation: check tracker existence first,
  then verify credentials separately
* Add TrackerExists() method for public tracker access checking
* Improve error messages to distinguish between "tracker doesn't
  exist" vs "invalid credentials"
* Add todo.sr.ht example to bridge new command help documentation
* Update mock client to implement the new interface

Users can now use the more intuitive --url flag with full
todo.sr.ht URLs and receive clearer feedback when configuration
fails.

Signed-off-by: Matěj Cepl <mcepl@cepl.eu>

gokart seems to be completely dead these days

1. GraphQL query restructuring: Fixed queries to use me.trackers and
   me.tracker structure
2. Submitter field handling: Changed to *json.RawMessage with new
   GetSubmitter() method for proper type unmarshaling
3. API parameter changes: GetTickets() now uses trackerName instead of
   trackerID
4. URL parsing fix: Extract only tracker name from owner-prefixed URLs
5. Error handling: Added tracker validation and better submitter parsing

…r access

- Replace me.trackers query with tracker(name: String!) query  
- Add getTrackerByNameQuery for public tracker access
- Fixes blocking bug preventing bridge configuration for public trackers

Resolves issue where users couldn't access trackers they don't own

- Add CreateLabel, DeleteLabel, AssignUser, UnassignUser methods
- Completes client interface for full todo.sr.ht API coverage
- Enables label creation and user assignment functionality

- Add CreateLabel, DeleteLabel, AssignUser, UnassignUser mock methods
- Completes mock client interface for full API coverage

- Add createTodoSRHTLabel helper function with validation
- Auto-create missing labels during export with default colors  
- Improve error messages for label operations
- Add input validation for label names
- Enhance error handling with detailed messages

Resolves issue where missing labels caused export failures

- Handle Assignment events in ensureEvent function
- Map assigner and assignee to git-bug identities  
- Emit warnings for assignment events (git-bug has no native assignment)
- Completes import coverage for all todo.sr.ht event types

- Add json import for JSON marshaling
- Fix mock function signatures to match interface
- Convert User entities to JSON raw messages for API compatibility
- Resolves compilation errors in test suite

- Add TestTrackerExists for public tracker access validation
- Add TestCreateLabel for label creation functionality  
- Add TestAssignUser for user assignment operations
- Add TestDeleteLabel for label deletion operations
- Provides complete test coverage for all new client methods

- Add MockCreateLabel, MockDeleteLabel, MockAssignUser, MockUnassignUser
- Update mock client to support all new GraphQL mutations
- Enables testing of label creation and user assignment functionality

- Add comprehensive feature list with import/export capabilities
- Document new label creation and user assignment functionality  
- Add troubleshooting section for common issues
- Include usage examples for all major operations
- Document advanced features and error handling improvements

The GraphQL query for GetEvents was updated to correctly traverse the
schema via `me.tracker.ticket` instead of a direct top-level `ticket`
query. This change aligns the client with the current todo.sr.ht API
structure.

The `GetEvents` function signature and its mock implementation were updated
to accept `trackerName` as a parameter. Corresponding call sites in
`import.go` and `import_test.go` were also adjusted to pass this new parameter.

A minor whitespace adjustment was also made in `doc/usage/todosrht.md`.