feat(picker): Enter is the remappable jump Action (#63)

[hammadmajid]

What changed

• internal/action: register the built-in jump Action (key = "enter", jump = true, no run).
• internal/picker/update.go: delete the three hard-coded enter cases and choose(). Enter now dispatches through the Action map like any other key, in the default key map, the vim list focus and the vim filter focus.
• Docs: README (key tables, remapping Enter), CONTEXT.md, and the Choice/ChosenAction comments.

Behaviour notes

• jump can be moved ([actions.jump] key = "alt+enter") or unbound (key = ""). Another Action can then take enter.
• Taking enter for another Action while jump still holds it is the existing duplicate-key error from feat: Actions — run a command on the selected Project with a key #62, not an automatic unbinding. The README says to move or unbind jump first.
• Enter on an empty list is now a no-op (the Action dispatch needs a row); it used to quit with no choice. Esc still cancels.
• The footer key hints still say enter jump; feat(picker): key-hint footer and vim ? overlay #67 makes them follow the bindings.
• config.example.toml does not exist yet (feat(cli): example config in releases and cdd config init/path/edit #70), so the keys are not added there.

Verification

• gofmt -l . empty, go vet ./... clean, go test ./... passes except the known fish cases of TestScriptExecution.
• New tests: built-in jump on enter; merge with jump moved/unbound and enter given to another Action; picker Enter Jumps in default, vim list and vim filter focus; unbound enter does nothing; remapped enter runs the other Action and jump Jumps on its new key.
• Built the binary and ran it with a temporary XDG_CONFIG_HOME: configs moving jump and giving enter to code, unbinding jump, and no config all load and reach the Picker (which then needs a TTY); code on enter with jump untouched fails with the line-numbered error actions.code: key: "enter" is already bound to "jump".

Closes #63