Remove picocolors dependency in favor of a native util.styleText facade

[shouze]

Remove picocolors dependency in favor of a native util.styleText facade

Context

picocolors is currently the only styling dependency in the codebase
(AGENTS.md: "picocolors is the only styling dependency; do not add chalk or
similar"). It is imported directly in 9 production files: src/render.ts,
src/render/highlight.ts, src/render/summary.ts, src/render/team-pick.ts,
src/tui.ts, src/upgrade.ts, src/api.ts and github-code-search.ts, plus a
few test files (src/render/terminal.test.ts, src/render/team-pick.test.ts)
and src/test-setup.ts (which forces FORCE_COLOR=1 for ANSI-assertion tests).

Since Node.js 20.12 / Bun 1.4, util.styleText (from node:util) ships a
built-in, zero-dependency ANSI styling API with automatic TTY / NO_COLOR /
FORCE_COLOR detection, matching the exact style names picocolors exposes
(dim, bold, italic, underline, and the named colors/background colors).
This removes the last external styling dependency from the project.

The only style names actually used today are: dim, bold, italic,
underline, red, green, yellow, cyan, magenta, white, black,
bgMagenta. Several call sites compose two or three styles by nesting
picocolors calls, e.g. pc.bold(pc.yellow(s)),
pc.bgMagenta(pc.black(pc.bold(" x "))), pc.cyan(pc.underline(url)). bold
and dim share the same SGR reset code (22), so naively replacing nested
picocolors calls with two chained util.styleText() calls is not guaranteed to
be byte-identical — picocolors has internal nesting-safe logic that a naive
facade would lose. util.styleText natively accepts an array of formats in a
single call (styleText(['bold', 'yellow'], text)), which is the correct way to
reproduce these compositions safely.

Typing gap found during implementation: the @types/node version resolved
in this project (25.2.3, pulled transitively via bun-types) does not declare
styleText on the node:util module, even though Bun 1.4's runtime implements
it. bun run/bun test still work at runtime (Bun strips types, it doesn't
type-check), but editors/IDEs would show a type error on the import. An ambient
module augmentation is required to fill this gap until @types/node catches
up.

Decision

Remove picocolors entirely and introduce a single new module, src/style.ts,
as the sole call site for node:util's styleText in the codebase — the
same convention already used by src/render/terminal.ts for Bun-native
terminal APIs (Bun.stringWidth, Bun.stripANSI, Bun.sliceAnsi). All 9
production files import from this facade instead of picocolors directly. A
small ambient .d.ts augmentation ships alongside the facade to type
styleText until upstream @types/node declares it.

Solution

1. Add an ambient module augmentation (e.g. src/style.d.ts) declaring
   styleText's signature on node:util, since @types/node 25.2.3 does not
   expose it yet.
2. Create src/style.ts:
   • Export one function per single style actually in use: dim, bold,
     italic, underline, red, green, yellow, cyan, magenta,
     white, black, bgMagenta — each a thin wrapper around
     util.styleText('<name>', text).
   • Export a style(names: StyleName[], text: string): string composer for
     multi-style call sites, using util.styleText(names, text) in a single
     call (not nested wrapper calls), to avoid the bold/dim reset-code
     collision and produce a single clean SGR sequence.
   • No other module may import node:util's styleText directly — this
     mirrors the existing render/terminal.ts convention and must be
     documented as such in AGENTS.md.
3. Add src/style.test.ts:
   • For every style name and every composed combination found in the
     codebase, assert the exact ANSI byte sequence produced by src/style.ts
     matches what picocolors produced before the migration (golden-master
     comparison) — this specifically guards against the bold/dim nesting
     collision risk described above.
   • Test NO_COLOR, FORCE_COLOR, and non-TTY stdout behavior explicitly.
4. Migrate all 9 production files to import from src/style.ts instead of
   picocolors, replacing:
   • Single-style calls 1:1 (pc.dim(x) → style.dim(x), etc.)
   • Nested composition calls with the array composer (pc.bold(pc.yellow(s))
     → style.style(["bold", "yellow"], s))
   • In github-code-search.ts, the existing manual HAS_COLOR gate for
     Commander's configureHelp styling hooks keeps its explicit TTY check
     but calls into src/style.ts instead of picocolors.
5. Update the test files that reference picocolors directly
   (src/render/terminal.test.ts, src/render/team-pick.test.ts) to use the
   new facade, and update stale comments in src/render/highlight.test.ts and
   src/test-setup.ts.
6. Verify FORCE_COLOR=1 set in src/test-setup.ts still forces ANSI output
   under bun test even though stdout is typically piped in that context.
7. Remove picocolors from package.json dependencies and run
   bun install to update bun.lock.
8. Update documentation: AGENTS.md (the "picocolors is the only styling
   dependency" note) and .github/skills/documentation.md (the "only
   picocolors (CLI) and VitePress built-ins" note) to describe the new
   src/style.ts convention.

Acceptance Criteria

• No file in the repository imports picocolors (bun run knip reports it
  as unused, and it is removed from package.json)
• src/style.ts is the only module importing styleText from node:util
• All ANSI output (TUI, help text, progress bars, error messages) is
  visually identical to before the migration when tested manually in a
  real terminal
• NO_COLOR=1 disables all color output; FORCE_COLOR=1 forces color
  output even when stdout is piped (verified both manually and by a test)
• Golden-master tests in src/style.test.ts pass for every style and every
  composed combination found in the codebase, proving no regression from
  the bold/dim reset-code nesting risk
• bun test, bun run lint, bun run format:check, bun run knip, and
  bun run build.ts all pass

Definition of Done

• PR merged to main following the standard refactor workflow (branch
  refactor/remove-picocolors, signed commits, behavior-preserving note in
  the PR description)
• picocolors no longer appears in package.json, bun.lock, or anywhere
  in src/, github-code-search.ts
• AGENTS.md and .github/skills/documentation.md reflect the new
  src/style.ts convention
• No follow-up regression reported in manual TUI testing across at least
  one real terminal session, one piped/non-TTY run, and one
  NO_COLOR/FORCE_COLOR override run