Feat: add support for multiple-repositories, show a repos list when lazygit starts in a directory of repos

[janosmiko]

PR Description

When you start lazygit in a directory that is not a git repo but contains repos, lazygit now lists those repos in a new Repositories panel. Before this change, lazygit showed the "not a repository" prompt and it still shows if there's no repos in the subdirs.

• Discovery walks down multiRepo.maxDepth levels (default 2). It stops at each repo, so it does not list nested repos or submodules. It skips hidden directories and symlinks.
• Lazygit opens the first valid repo (sorted by path) and focuses the list. If a repo is broken, for example an empty .git or a stale worktree .git file, lazygit skips it.
• The Repositories panel replaces the Status panel in window 1. repos is a new valid tab name for gui.sidePanels. If your config lists repos already, lazygit uses it as is.
• Each row shows the path relative to the start directory, the branch, and * if the working tree has changes. The open repo has a marker, like the current worktree in the Worktrees panel.
• space or enter switches to the selected repo. Focus stays on the list. The main view shows git status for the selected repo. e and u from the Status panel work here too, and ctrl+r still opens recent repos.
• If lazygit finds no repos, the existing notARepository behavior runs. GIT_DIR and -p work as before.

Performance:

• Lazygit reads branch names from HEAD files. Dirty checks run git status --porcelain in the background, 8 at a time, with GIT_OPTIONAL_LOCKS=0. So they never take index.lock in a repo that you work in, and they do not block a repo switch.
• The full list reloads at startup, on a repo switch, and on R. Other refreshes update only the row of the open repo. The periodic files refresh does not touch the list.

Out of scope for this PR: ahead/behind counts, rebase/merge state per repo, and bulk fetch or pull across repos.

Known limitation: a repo switch clears the list filter. All other panels behave the same way on a repo switch.

Open question: I replaced the Status panel in this mode because Status shows only the current repo. If you prefer the list as a separate tab next to Status, I can change it, but I think this is a pretty great UX.

Please check if the PR fulfills these requirements

• Cheatsheets are up-to-date (run go generate ./...)
• Code has been formatted (see here)
• Tests have been added/updated (see here for the integration test guide)
• Text is internationalised (see here)
• If a new UserConfig entry was added, make sure it can be hot-reloaded (see here)
• Docs have been updated if necessary
• You've read through your own file changes for silly mistakes etc

I checked these facts against commit e9d5155:

• go generate added the "Repositories" section to every docs-master/keybindings/*.md and the new key to Config.md.
• The loader reads multiRepo.maxDepth from the config at each full reload, so a changed value takes effect on the next R.
• The last checkbox is your own read-through, so I left it unchecked.