fix(worktree): include linked issue in task branch names

[dawNotPoi]

Summary

• name new forge issue worktrees with the provider and issue number, for example task/github-issue-152-2
• preserve legacy naming for ordinary tasks, pull requests, and invalid source metadata
• keep retry and recreation behavior stable while sanitizing and bounding directory names
• cover the behavior with helper cases and a real Git worktree test, including existing github and gitlab leaf branches

Testing

• cargo test --no-default-features --bin codeg-server --lib (3254 passed, 1 ignored)
• cargo check --no-default-features --bin codeg-server
• cargo clippy --no-default-features --bin codeg-server --lib -- -D warnings
• cargo check
• cargo clippy --all-targets --features test-utils -- -D warnings
• cargo check --no-default-features --bin codeg-mcp
• cargo clippy --no-default-features --bin codeg-mcp -- -D warnings

The full desktop test command was also attempted locally, but the integration-test link exceeded the 15 GiB WSL memory limit. Desktop check and all-target clippy completed successfully.

Fixes #619