🐛 bug(various fixes): Improve error handling, UX, and robustness

[iamrajjoshi]

Summary

• Fix silent post-checkout hook errors — runPostCheckoutHook now warns via ui.Warn when the hook fails instead of silently swallowing the error
• Fix HasUnpushedCommits swallowing all errors — only treats "no upstream" as unpushed; real git errors (network, permission, corruption) are now propagated
• Fix branch name collision from slash stripping — feature/auth now creates directory feature-auth instead of featureauth, preventing collisions with a branch literally named featureauth
• Add cleanup on partial clone failure — if any step after the bare clone fails (fetch, config, worktree creation), both bareDir and worktreesDir are cleaned up
• Add interactive worktree picker for pwd — when called with no argument, shows a numbered list and prompts for selection (also makes wwg with no args work interactively)
• Expose postCheckoutHook in ww config CLI — now visible in --list, gettable and settable via ww config postCheckoutHook <path>
• Improve run --all result aggregation — tracks failures and returns a summary error like command failed in 2/5 worktrees: main, payments

Also verified:

• Error wrapping already uses %w consistently (no %v found)
• Shell completion is correctly using --generate-shell-completion per urfave/cli/v3 API

Test plan

• All 34 existing + new tests pass (go test ./...)
• go vet ./... clean
• Manual verification: ww config --list shows postCheckoutHook
• Manual verification: --generate-shell-completion returns all subcommands
• Manual verification: shell-init detects bash/zsh correctly
• Integration test for slashed branch directory naming (TestNew_SlashedBranchDirName)
• Integration test for postCheckoutHook config set (TestConfig_PostCheckoutHook)
• Unit test for empty worktree picker (TestPickWorktree_EmptyList)