feat: add dynamic shell autocompletion for branches and tickets

[developerdavi]

Summary

• Adds a hidden _completions command that outputs tracked branch names or ticket IDs (one per line) by reading ~/.morg/ state files
• Updates Bash and Zsh completion scripts to call morg _completions dynamically for commands that accept branch/ticket arguments
• Completions are silent on errors (non-morg repos, missing files) and require no config validation or network calls

Commands with dynamic completion

Command	Completes
switch, complete, delete, untrack	Branch names (active/pr_open)
track	1st arg: branches, 2nd arg: ticket IDs
pr view	Branch names
ticket, tickets	Ticket IDs + existing flags

Test plan

• pnpm typecheck — clean
• pnpm lint — clean
• pnpm test — 60 tests passing (6 new)
• morg _completions branches outputs tracked branches
• morg _completions tickets outputs ticket IDs
• Silent exit in non-morg directories
• Manual: eval "$(morg shell-init zsh)" then morg switch [tab] shows branches