test: add command-level tests for create, sync and config

[epodivilov]

Adds command-level tests for the three CLI commands that had none. Tests only — no production code changed. They follow the existing update.test.ts / remove.test.ts pattern: drive the command through fake ports and assert on what it renders via UiPort plus the resulting fake git/fs/shell state.

Covered

create (19 tests)

• Happy path: worktree creation with the resolved base, config and local-config symlinks, file copy, declared symlinks, post-create hook command/cwd/env.
• Branch resolution: explicit --base, base prompt when the flag is absent, existing local branch (no base resolution), remote-only branch (checked out from the remote).
• --dry-run: every preview line, and no git/fs/shell side effects.
• Failures: branch already has a worktree, target directory exists but is not a worktree, git refuses to create the worktree, missing branch name in non-interactive mode.
• Non-fatal degradations: config symlink failure, copy failure, symlink failure, failing hook, missing config — each warns and the command still completes.
• Rollback: an interrupt while the worktree is being populated triggers the registered CleanupHandle, which force-removes the worktree and cancels with exit 130. The rollback's own failure is swallowed and never reaches the user. A successful run deregisters the handler.

sync (16 tests)

• Happy path across multiple worktrees, up-to-date reporting, broken symlink recreation, branch filter, empty worktree list.
• --dry-run: "would" verbs, no writes, hooks skipped.
• Existing destinations: skip with the --force hint, overwrite with --force, real file where a symlink belongs.
• post-sync hooks: command/cwd/env per worktree, failing hook downgraded to a warning.
• Failures: unknown branch, outside a git repository, missing config, legacy config warning.

config show (8 tests)

• Human output: sources header, per-field provenance for repo/local/global/default, repo winning over global.
• --json: payload shape on stdout, undefined serialized as null, interactive UI bypassed.
• Errors: missing config and invalid JSONC on the human path, JSON error on stderr with a failure exit.

Not covered

• Cancel paths (Ctrl-C at the branch/base prompts) in resolve-params.
• Glob-based copy/symlink patterns and tracked-target skipping — these live in resolve-worktree-plan and are exercised at the use-case level.
• The parent config command without a subcommand (citty help routing).
• In config show --json, the catch around the JSON branch is only reachable because the test stubs process.exit instead of terminating, so only the first stderr line is asserted.