feat(picker): Enter is the remappable jump Action

[hammadmajid]

Part of #61. Covers feature 2 (remap Enter).

What

Make Jump the built-in Action jump, bound to enter by default. It can be rebound or overridden by name like any Action:

[actions.jump]
key = "alt+enter"      # Jump moves here

[actions.code]
key = "enter"          # Enter now opens VS Code
run = "code {path}"
detach = true

• Built-in Actions are overridden by name: [actions.jump] changes only the fields it sets.
• Rebinding enter without moving jump leaves jump unbound. That is allowed, and the key hints (feat(picker): key-hint footer and vim ? overlay #67) simply won't list it.
• Applies in both the default and vim key maps, and in the vim filter focus.

Acceptance

• enter is no longer hard-coded in internal/picker/update.go; it dispatches through the Action map.
• Tests: default Enter Jumps; remapped Enter runs the other Action; jump on another key Jumps.
• README documents remapping Enter.

[hammadmajid]

Follow-up: PR #77 made a user Action on a built-in's key (e.g. [actions.code] key = "enter") a config error. That is against this ticket and against map decision Q12: a user binding wins, and the built-in it displaces is left unbound. Two user Actions on one key stays an error. Fixed in a follow-up PR.