Add dynamic shell completion for aliases and config keys

[jongio]

Problem

The shell completion scripts only complete static tokens (command names, flags, shell names, config subcommands). They cannot complete the values that matter most day to day: session aliases for dispatch open, or config keys for dispatch config get/set/unset. You still have to type or paste those by hand.

Proposed solution

Add a hidden dispatch __complete <what> command that prints newline-separated candidates for a given kind, then wire the four completion scripts to call it for the right argument positions. Kinds to support:

• aliases reads the configured session aliases.
• config-keys lists the settable config keys.
• shells lists the supported completion shells.

Completion stays fast because each kind reads only local config or a static list. Unknown kinds print nothing and exit zero so a completer never breaks.

Acceptance criteria

• dispatch open <TAB> completes configured session aliases in bash, zsh, fish, and PowerShell.
• dispatch config get <TAB> (and set/unset) completes config keys.
• dispatch completion <TAB> still completes the four shells, now sourced from __complete.
• dispatch __complete <unknown> prints nothing and exits zero.
• The __complete command is not advertised in help or usage.
• README notes that completion includes dynamic values and tests cover each kind and the unknown case.

Complexity

M

Priority

Medium