feat: implement the Picker (Bubble Tea v2, variant D)

[hammadmajid]

What

Implements the Picker: internal/picker, built on Bubble Tea v2 (charm.land/bubbletea/v2 v2.0.9, charm.land/lipgloss/v2 v2.0.6, charm.land/bubbles/v2 v2.2.1, github.com/sahilm/fuzzy v0.1.3, per the bubbletea-stack research).

• Run(rows []Row, status StatusFunc, opts Options) (Row, bool, error). rows must already be in History order; the Picker never computes that order itself. Draws on /dev/tty via tea.OpenTTY, falling back to stderr; stdout is never written.
• Row carries a Project (Kind, Name, absolute Path), LastVisit, and Visits. StatusFunc is called once per Row via one tea.Cmd each, fanned out through tea.Batch and bounded by a semaphore of 8, keyed by Project path.
• Options carries Vim and an initial Query.
• Look: the accepted "variant D, grouped + preview" prototype — filter line on top, left pane rows grouped under bold Kind headers (Kinds ordered by first appearance), selected row gets a caret and accent name (no background highlight), right pane preview box, footer with rule/legend/keys and the match count. Status glyph cluster and the light/dark palette match the accepted look exactly.
• Keys: default (typing filters, arrows/ctrl+p/ctrl+n move, enter chooses, esc cancels, ctrl+u clears) and vim (list focused on open, j/k move, g/G jump to the ends, f or / focuses the filter, esc in the filter returns to the list keeping the query, esc/q on the list cancels).
• Narrow-terminal degradation: list width capped at 55% of the terminal, preview dropped under 30 remaining columns; relative time compresses to its short form before names truncate to a floor of 8; status cluster never shrinks; under 15 rows the legend line drops, under 10 the keys line too. Recomputed on every tea.WindowSizeMsg without losing state.
• Empty History renders the "run cdd scan to seed History" hint.

Split across picker.go (public API, Run), model.go (Model, Init, grouping/filtering), update.go (key maps), view.go (rendering), styles.go (palette, glyphs), relative.go (relative-time formatting), layout.go (width/height degradation) — every file well under 300 lines.

Why

Closes #20.

Testing

go build ./... && go vet ./... && go test ./... all pass. Tests are table-driven, stdlib-only, in package picker_test, and exercise: Model.Update with tea.KeyPressMsg and status messages via a fake StatusFunc, Kind-group ordering, fuzzy filtering, relative-time formatting, and the width/height degradation rules — all without a real terminal.

A handful of small helpers (Model.Chosen, RelativeTime/RelativeTimeShort, Layout/ComputeLayout) were exported specifically so the external test package could exercise them directly, per this repo's testing convention.

Notes for the reviewer

• No CLI wiring yet: this ticket is the Picker package itself. internal/jump (rows already ordered) and the cli/jump command that calls picker.Run and prints the chosen path are follow-ups (the ticket notes #27/#21 depend on this).
• [keys] vim config plumbing (config.toml) is not part of this ticket; Options.Vim is ready for a caller to set it.
• I did not manually verify the rendered output against a real TTY (no interactive terminal available in this environment); the look was built directly from the accepted prototype's source and the resolution issue's exact colours/glyphs/spacing rules, and is covered by the Update/layout/formatting tests instead.

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