feat: discover repositories from current checkout

[fschrhunt]

Problem

Repositories cloned directly with Git beneath repos/ were not convenient to use because new still required an explicit repository argument. Creating a task after cloning an empty repository also failed, and manually deleted task folders left stale Git metadata without an actionable cleanup instruction.

Solution

• Let zero-argument ws new discover the base repository containing the current directory when it lives directly beneath repos/.
• Continue discovering repositories and worktrees from the filesystem and Git metadata rather than adding a persistent index.
• Create an unborn task branch when a cloned repository has no commits.
• Keep manually deleted task folders out of ws list, while having ws doctor report stale metadata and recommend git worktree prune.
• Update help, documentation, the generated collection guide, and focused regression tests.

Validation

• make check
• 47 Bats tests pass
• bin/workspaces help

Release and safety notes

• No public command behavior changed.
• CHANGELOG.md has an Unreleased entry when user-visible behavior changed.
• No secrets, private infrastructure, or generated local state is included.

Public behavior intentionally changes by allowing workspaces new with no arguments from a base checkout.