feat: Actions, named commands bound to keys in the Picker (#62)

[hammadmajid]

What changed

• New internal/action package: the Action type, the built-in registry (builtins, empty for now), Merge (user [actions.<name>] tables override a built-in field by field), key validation and the Runner interface with ExecRunner.
• Config decodes [actions.*] and validates at load. Errors name the line: unknown key, esc and ctrl+c, a plain printable key without keys.vim, two Actions on one key, and a missing run.
• The Picker dispatches Action keys before its own key map, so a binding overrides a navigation key. Plain printable keys only fire in vim list focus. A detached Action starts through the injected Runner and the Picker stays open; a failure to start shows on the footer's key line until the next key press. Any other Action quits the Picker with the Action in the new picker.Choice.
• jump.Resolve runs a non-detached Action after the Picker quits, on /dev/tty for all three streams, records a Visit, prints the path only for jump = true with a zero exit, and passes a non-zero status on via jump.ExitError (the CLI exits with it, silently). Detached Actions record their Visit through a recording Runner wrapper.
• CONTEXT.md defines Action; the README documents Actions.
• Enter is unchanged (feat(picker): Enter is the remappable jump Action #63 moves it onto the Action map).

Verification

• gofmt -l . empty, go vet ./... clean, go test ./... passes except the known fish cases of TestScriptExecution.
• Tests: key and merge tables (internal/action), config line numbers (internal/config), key dispatch with a fake runner including the footer message (internal/picker), the Resolve flows with a fake runner (internal/jump), and ExecRunner against a fake tty file (path with a space and a quote, CDD_PATH, cwd, exit status, detached start and start failure).
• Smoke test with a temporary XDG_CONFIG_HOME: a config binding ctrl+c fails with line 8: actions.bad: key: "ctrl+c" can never be bound; a valid one loads.

Closes #62