feat: config option to use the fzf-style list layout (prototype variant C) for the Picker

[hammadmajid]

What problem are you trying to solve?

The Picker ships one layout: prototype variant D, rows grouped under Kind headers with a preview pane. Some users, and some terminal sizes, suit a flat fzf-style list better: no Kind headers taking vertical space, the prompt at the bottom where fzf users expect it, and a background-highlighted selected row.

Proposed solution

Add a config key that selects the Picker layout, defaulting to the current one:

[picker]
layout = "grouped"   # default, prototype variant D
# layout = "list"    # prototype variant C: flat list + preview

list renders prototype variant C · List + preview from the prototype/picker-look branch (prototype/picker/variant_c.go): a flat list ordered by History with never-visited Projects last, kind/ shown muted before each Project name, one status glyph per row, relative time right-aligned, the filter prompt below the list, a ▌ bar and background highlight on the selected row, and the same boxed preview pane on the right. Fuzzy matching, key maps (default and vim), status glyphs, colours, and the narrow-terminal rules stay shared with the grouped layout.

Alternatives considered

• A runtime toggle key instead of a config key. A config key is the decided pattern ([keys] vim), and a layout is a standing preference rather than a per-session choice.
• Making list the default. The grouped layout was the one accepted in the prototype review, so it stays the default.

Additional context

• Prototype: https://github.com/kryft-dev/cdd/tree/prototype/picker-look/prototype/picker (run go run ./prototype/picker -variant c on that branch).
• The accepted look for the current layout is recorded on the closed wayfinder ticket Picker look: runnable mock to react to.
• Unknown values of layout should be a config error, matching how every other key is validated.