feat(workspace): loom reset command to discard work and rebase to origin/main

[BalduinLandolt]

Summary

Add a loom reset command that discards all local changes in a workspace, fetches from origin, and rebases all workspace branches onto origin/main. This provides a clean way to start fresh without tearing down and recreating the workspace.

Motivation

When working in a workspace, branches can drift from origin/main over time, or you may want to abandon the current direction and start over. Currently, the only way to get a clean slate is to loom down the workspace and recreate it with loom new. A dedicated reset command would be faster and preserve the workspace structure.

This is especially useful when:

• An agent has made changes you want to fully discard
• origin/main has moved significantly and you want all worktrees rebased
• You want to restart work on a feature from a clean state without recreating the workspace

Commands affected

• loom init
• loom new
• loom add
• loom remove
• loom down
• loom list
• loom status
• loom exec
• loom shell
• loom save
• loom open
• loom registry
• TUI
• New command

Modules affected

• cli
• workspace
• git

Proposed behavior

loom reset [name]

• No argument: resets the workspace detected from the current directory
• With <name>: resets the named workspace

For each repository worktree in the workspace, the command should:

1. Discard all uncommitted changes (staged and unstaged)
2. Fetch origin
3. Hard-reset / rebase the worktree branch onto origin/main (or the configured default branch)

Before executing, the command should show a confirmation prompt warning that all local work will be lost — e.g.:

⚠ This will discard ALL uncommitted changes and reset branches to origin/main.
  Workspace: my-feature
  Repos: repo-a, repo-b, repo-c

  Continue? [y/N]

A --force flag could skip the confirmation for scripting use cases.

If progress indication is available (see #50), it should be used here as well.

Acceptance criteria

• loom reset resets the workspace detected from the current directory
• loom reset <name> resets a named workspace
• All uncommitted changes (staged and unstaged) are discarded in every worktree
• origin is fetched for each repository
• Worktree branches are rebased onto origin/main (or the repo's default branch)
• A confirmation prompt is shown before destructive action
• --force skips the confirmation prompt
• Clear error message if the workspace is not found

[subotic]

Closed by #69 — loom reset command with base_branch support