fix(action): a user binding displaces the built-in on its key (#63)

[hammadmajid]

What changed

• action.Merge no longer rejects a user Action on a key a built-in holds by default. The user binding wins and the built-in is left unbound, as if its key were "". So [actions.code] key = "enter" alone now works.
• A user rebinding one built-in onto another built-in's key displaces that one too ([actions.remote] key = "enter" unbinds jump).
• Two user-bound Actions on one key (and two built-ins both on a default key) is still an error; esc and ctrl+c still can't be bound.
• Reversed the feat: Actions — run a command on the selected Project with a key #62 test that asserted a built-in clash is an error; replaced the feat(picker): Enter is the remappable jump Action #63 test that required moving jump first.
• README: remapping Enter alone is enough.

Verification

• New tests: Enter displacement, built-in rebound onto a built-in's key (both orders), two user Actions on enter fail.
• gofmt -l . empty, go vet ./... clean, go test ./... passes except the known fish cases of TestScriptExecution (no TTY here).
• Loaded a config with only [actions.code] key = "enter" run = "true" through config.Load with a temporary XDG_CONFIG_HOME (throwaway test, not committed): jump unbound, code on enter. Binary builds.

Refs #63