feat(picker): forget the selected Project with ctrl+d (#69)

[hammadmajid]

What changed

• New built-in Action forget on ctrl+d. It asks forget <name>? y/n on the footer; y calls the Store's Forget, drops the row and keeps the cursor at the same index (the row below, or the one above at the end of the list). Any other key cancels and is consumed, so y/n never reach the filter.
• A Forget error shows red on the footer and keeps the row.
• Action.Copy bool is generalised to Action.Internal string (copy, forget): a Picker-side job with no run, cleared by a user run or jump, rebindable and unbindable like any Action.
• picker.Options.Forget is wired to store.Forget in jump.Resolve; the vim help overlay and README describe it.

Verification

• Model tests: confirm, cancel (several keys), cursor after removing the first, middle, last and only row, a filtered list, a Forget error, no Forget func, a rebound key, the vim help overlay.
• gofmt -l . empty, go vet ./... clean, golangci-lint run ./... 0 issues, go test ./... passes except the known local-only fish cases of TestScriptExecution; the binary builds.

Closes #69