Let embedders register built-in themes via RegisterBuiltinThemes

[EronWright]

What

Adds styles.RegisterBuiltinThemes(fsys fs.FS) error, which lets an embedder
contribute additional built-in themes from a filesystem (typically an
embed.FS of themes/*.yaml).

Why

Themes are already partial overrides on top of DefaultTheme() — loadBuiltinTheme
and loadThemeFrom both mergeTheme(DefaultTheme(), &override), which is why a
bundled theme YAML only declares the colors it changes. But the built-in set is a
single compile-time //go:embed themes/*.yaml, which a downstream consumer can't
extend.

A downstream CLI/TUI built on cagent wants its own theme(s) to be first-class
without forking. RegisterBuiltinThemes generalizes the bundled-theme source
from one hardcoded embed.FS to a list, so registered themes flow through the
exact list/load/merge pipeline cagent already uses. They are treated like
cagent's own built-ins with no changes to the picker or handlers:

• appear in ListThemeRefs (and the /theme picker),
• resolve via LoadTheme / ApplyThemeRef,
• classify as built-in (IsBuiltinTheme),
• persist as the user's selection.

Registered sources take precedence over the bundled themes, so an embedder can
override a built-in and, in particular, mask default with their own. A
masked default is still a partial override merged onto cagent's pristine
DefaultTheme() base, so it only needs to specify the fields it changes.
DefaultTheme() itself remains the canonical merge base.

Embedder usage

import (
    "embed"

    "github.com/docker/docker-agent/pkg/tui/styles"
)

//go:embed themes/*.yaml
var brandThemes embed.FS

// At startup, before applying any persisted theme:
if err := styles.RegisterBuiltinThemes(brandThemes); err != nil {
    return err
}

themes/brand.yaml is a partial override; unset fields fall back to the default
theme:

# themes/brand.yaml
name: Brand
colors:
  accent: "#FF6A00"
  brand: "#FF6A00"
  background: "#1A0F0A"
# text_primary, success, error, chroma.*, etc. are inherited from the default.

To make the brand theme the launch default, ship it as themes/default.yaml
instead (or in addition) — it masks cagent's default while inheriting everything
it doesn't set.

Notes / semantics

• The source must contain themes/<ref>.yaml (or .yml). An embedder that
  embeds at themes/*.yaml passes its embed.FS directly; otherwise fs.Sub to
  re-root at the themes dir.
• A registered ref that collides with a bundled name overrides it; the bundled
  name is still listed once.
• RegisterBuiltinThemes validates the source eagerly (nil / missing themes
  dir) so problems surface at registration, not at picker time.

Tests

Tests live in theme_test.go (package styles), consistent with the existing
theme tests, and exercise registration through the public functions an embedder
uses, with a small unexported reset for isolation:

• TestRegisterBuiltinThemes_EmbedderFlow — registers from a real embed.FS and
  runs the full register → list → IsBuiltinTheme → ApplyThemeRef loop.
• TestRegisterBuiltinThemes — listed, classified built-in, loads merged onto
  the default (overridden field changes, unset fields inherited).
• TestRegisterBuiltinThemes_MultipleSources — aggregates across more than one
  source; first-registered wins a collision between two registered sources.
• TestRegisterBuiltinThemes_OverridesBuiltin — a registered name overrides the
  bundled theme.
• TestRegisterBuiltinThemes_MasksDefault — masks default, merged onto the
  pristine DefaultTheme() base.
• TestRegisterBuiltinThemes_Errors — nil / missing-themes-dir rejected eagerly.

Full pkg/tui/styles suite passes under -race; go vet and gofmt clean.

[Sayt-0]

Pushed a few fixes (7cad5565):

• Lint: errors.New + require.Error to get CI green (perfsprint / testifylint).
• Docs: the godoc said default is ignored and collisions keep the bundled theme, but registered sources actually take precedence and can mask default (as the PR description and the override/mask tests assert). Rewrote it to match.
• Bug: registration only invalidated the refs-list cache, not the parsed-theme cache. A built-in (or default) loaded before RegisterBuiltinThemes stayed cached, so the override was silently dropped. Added InvalidateThemeCache("") + a regression test.