feat: macOS support (BSD date/stat, locale-safe rendering)

[xinnaider]

What

Adds macOS support, implementing the proposal in #2.

tokenline.sh abstracts the GNU-only calls behind two helpers that branch on a
one-time behavior probe (not uname, so Homebrew coreutils is picked up
automatically):

• epoch_from_iso() — GNU date -d vs BSD date -u -j -f "%Y-%m-%dT%H:%M:%S"
  (first 19 chars of the ISO string; transcript timestamps are UTC).
• file_mtime() — GNU stat -c %Y vs BSD stat -f %m.

On Linux the probe selects the GNU branch, which runs the original commands
verbatim — no behavior change.

It also pins LC_ALL=C. A comma-decimal locale (e.g. pt_BR) otherwise makes
awk/printf emit 46,2k and reject dotted input, diverging from the C-locale
CI. LC_ALL is used (not LC_NUMERIC) because it wins when a user exports
LC_ALL. Output is ASCII/bytes, so this is safe.

mapfile still needs bash 4+, so macOS users brew install bash — documented in
the README and surfaced by install.sh.

Changes

• tokenline.sh: GNU/BSD probe, epoch_from_iso, file_mtime, LC_ALL=C.
• install.sh: date/stat checks accept BSD (GNU-first); macOS hints.
• src/infra/system.ts: checkPlatform() accepts darwin.
• README / AGENTS.md: requirements, troubleshooting, roadmap, platform badge.
• changeset (minor).

Testing

• ShellCheck -s bash clean on tokenline.sh and install.sh.
• pnpm lint, pnpm typecheck, pnpm build pass.
• Rendered on macOS (bash 5.x, BSD date/stat): same fixture → same lines as
  Linux, including under a hostile LC_ALL=pt_BR.UTF-8.
• install.sh passes its dependency check on macOS with Homebrew bash + BSD
  coreutils.

Closes #2

[xinnaider]

Suggested labels (couldn't self-apply from a fork): platform:macos, area:statusline, area:installer, feature, effort:M.

[ropdias]

Thanks for the really thorough work here, @xinnaider, excellent first contribution! 🙌

It cleanly implements #2: probing behavior instead of uname (so Homebrew
coreutils is picked up automatically), keeping the GNU branch verbatim so Linux
output is unchanged, and the LC_ALL=C pin is a great catch for comma-decimal
locales.

I reviewed and verified locally:

• ShellCheck -s bash clean on both tokenline.sh and install.sh
• Same fixture → identical rendered output vs main on Linux (the macOS
  acceptance criterion)

One low-severity edge case I want to track separately: the BSD parse in
epoch_from_iso() uses a fixed %Y-%m-%dT%H:%M:%S mask, guaranteed for Claude
Code's .timestamp but unverified for Antigravity's .created_at. It degrades
gracefully (mtime fallback) and doesn't touch Linux, so I'm merging as-is and
opening a follow-up issue for it.

Approving, thank you again! 🚀