refactor(git): consolidate PR/MR resolution into unified remote_ref module (#893)

* refactor(git): consolidate PR/MR resolution into unified remote_ref module

Replace duplicate pr_ref.rs (520 lines) and mr_ref.rs (715 lines) with a
trait-based architecture in remote_ref/. Both platforms now share:

- RemoteRefProvider trait defining the platform-agnostic interface
- RemoteRefInfo struct with platform-specific data in PlatformData enum
- Unified resolve_remote_ref() in switch.rs

Also:
- Add ref_path field to CreationMethod::ForkRef to avoid re-deriving
- Add -- separators to git fetch/worktree commands for security
- Update module docs to reflect current API

Net reduction: ~420 lines. Adding new platforms (e.g., Bitbucket) now
requires only implementing RemoteRefProvider.

Co-Authored-By: Claude <noreply@anthropic.com>

* fix(gitlab): defer project URL fetching for ~1s perf improvement

GitLab fork MRs required 2 API calls (~500ms each) to fetch source and
target project URLs. These calls happened unconditionally, even when the
branch already tracked the MR (common case when re-switching).

Now:
- fetch_mr_info only fetches MR metadata (no project URL calls)
- fetch_gitlab_project_urls is called only after branch_tracks_ref fails
- For branches already configured, we skip ~1 second of API calls

Also fixes nested namespace display in source_ref output:
- extract_namespace_from_url now handles group/subgroup/repo correctly
- Example: git@gitlab.com:org/team/repo.git → org/team

Trade-off: Fork MR display now shows branch name only (not namespace:branch)
since fork_push_url isn't available until after the deferred fetch. The
full URL is still shown, and this only affects display, not functionality.

Co-Authored-By: Claude <noreply@anthropic.com>

* docs(gitlab): explain deferred URL fetching vs GitHub

Document why GitLab requires separate API calls for project URLs while
GitHub returns everything in one call. This explains the optimization
that defers URL fetching until after branch_tracks_ref check.

Co-Authored-By: Claude <noreply@anthropic.com>

* test(gitlab): add test for fetch_gitlab_project_urls validation

Cover the defensive check that rejects non-GitLab refs.

Co-Authored-By: Claude <noreply@anthropic.com>

---------

Co-authored-by: Claude <noreply@anthropic.com>

Author: max-sixty

src/commands/worktree/switch.rs

@@ -75,6 +75,8 @@ pub enum CreationMethod {
         ref_type: RefType,
         /// The PR/MR number.
         number: u32,
+        /// The ref path (e.g., "pull/123/head" or "merge-requests/42/head").
+        ref_path: String,
         /// URL to push to (the fork's URL). `None` when using a prefixed branch
         /// name (e.g., `contributor/main`) because push won't work.
         fork_push_url: Option<String>,

src/commands/worktree/types.rs

@@ -5,9 +5,7 @@ use std::path::PathBuf;
 // Submodules
 mod diff;
 mod error;
-pub mod mr_ref;
 mod parse;
-pub mod pr_ref;
 pub mod remote_ref;
 mod repository;
 mod url;