Consolidate 5 direct git2 usages outside git/ module

[Wirasm]

Problem

Five locations in kild-core use `git2` directly instead of going through the `git/` module's public API:

Location	Direct git2 call	What it needs
`forge/registry.rs:51`	`Repository::open` → reads remote URL	A `get_remote_url(path)` function
`sessions/destroy.rs:378`	`Repository::open` → `repo.remotes()`	A `has_remote_configured(path)` function
`sessions/info.rs:170`	`Repository::open` → `repo.statuses()`	Already exists: `get_worktree_status()`
`projects/types.rs:128`	`Repository::discover` → presence check	A `is_git_repo(path)` function
`cleanup/handler.rs` + `operations.rs`	`Repository::discover`, branch iteration	Functions for repo discovery and branch listing

These should be consolidated into the `git/` module's public API so that `git2` is only imported within `git/`.

Fix

1. Add missing public functions to `git/` module:
   • `has_remote_configured(path: &Path) -> bool` — absorbs `destroy.rs:378`
   • `get_remote_url(path: &Path) -> Option` — absorbs `forge/registry.rs:51`
   • `is_git_repo(path: &Path) -> Result` — absorbs `projects/types.rs:128`
   • `discover_repo_and_branches(path: &Path) -> ...` — absorbs `cleanup/` calls
2. Replace the 5 direct `git2::` usages with calls to these new functions
3. Remove `git2` from the import lists of `forge/registry.rs`, `sessions/destroy.rs`, `sessions/info.rs`, `projects/types.rs`, `cleanup/handler.rs`, `cleanup/operations.rs`

After this, `git2` is only imported within `git/` files — a prerequisite for clean `kild-git` crate extraction.

References

• Extract kild-git crate from kild-core #587 — Extract kild-git crate (this is a prerequisite step)
• Fix process/ ↔ sessions/ bidirectional coupling #589 — Remove duplicate git status check in sessions/info.rs (subset of this work)