feat: built-in copy Action copies the Project path (ctrl+y) (#66)

[hammadmajid]

Closes #66. Part of #61.

What

• New built-in copy Action on ctrl+y. It copies the Project's absolute path and the Picker stays open, with a green copied <path> on the footer line until the next key press (an error still shows red).
• action.ClipboardTool picks the first of wl-copy (only with WAYLAND_DISPLAY), xclip -selection clipboard, xsel --clipboard --input, pbcopy; action.Copy pipes the path to it.
• With none found, the Picker returns tea.SetClipboard(path), so the OSC 52 escape goes out through the running program's own terminal. A detached shell Action could not do that, so copy is handled Go-side via a new Action.Copy flag (set by the built-in only, not a TOML key). A user's run (or jump) clears it, so [actions.copy] run = ... overrides it and key rebinds or unbinds it.
• picker.Options.Copy is the injectable clipboard function (default action.Copy).
• README documents it.

Verification

• Tool order tested with fake PATHs and WAYLAND_DISPLAY; Copy piping, no tool, and tool failure tested.
• OSC 52 path tested by asserting the Cmd yields tea.setClipboardMsg carrying the path (the escape bytes are bubbletea's).
• gofmt, go vet, golangci-lint run ./... clean; go test ./... passes apart from the known local-only fish TestScriptExecution cases; binary builds.