Add 'config edit' to open the config file in your editor

[jongio]

What

Adds a dispatch config edit subcommand that opens the config file in your editor.

dispatch config edit

It writes the current config to disk first (so the file always exists, even on a
fresh install), opens it in your editor, and then re-reads the file after the
editor exits. If the edit left the file unparseable, the error is reported
instead of being silently applied.

Why

Today the only way to change settings is one config set key value call at a
time, or hunting down the file path with config path and opening it by hand.
For anyone who wants to review or change several settings at once, opening the
file directly is faster. This lines up with how tools like git config --edit
and gh config behave.

Editor resolution

1. $VISUAL
2. $EDITOR
3. Platform default: notepad on Windows, vi elsewhere

The editor value is split on whitespace, so entries like code --wait keep
their flags.

Changes

• New runConfigEdit handler plus resolveEditorArgv / launchEditor helpers in cmd/dispatch/config.go
• Help text updated in cmd/dispatch/main.go
• Shell completions (bash, zsh, powershell) updated in cmd/dispatch/cli.go
• README example added
• Tests covering success, argument rejection, editor failure, post-edit validation, and editor resolution

Testing

• go build ./...
• go test ./... -count=1
• go vet ./...
• golangci-lint run

All green.

Closes #254