fix: render extra usage segment + adopt clone-based install

[daniel3303]

Summary

• Fixes the extra segment never rendering once Claude Code began supplying rate_limits in the statusline stdin JSON (the common path).
• Makes the OAuth usage-API cache resilient to failed fetches so a transient error does not suppress retries for a full 60s window.
• Updates the install story: leads with "clone the repo, let Claude configure it", adds INSTALL.md so Claude has an authoritative step-by-step; paste-install stays as a fallback.

Code changes

statusline.sh + statusline.ps1

• Removed the effective_builtin gate on the usage-API fetch. Claude Code's stdin rate_limits only exposes five_hour / seven_day — extra_usage still has to come from https://api.anthropic.com/api/oauth/usage.
• Extracted extra-usage rendering into a helper (render_extra_usage / Format-ExtraUsage) used by both the builtin and API-fallback branches.
• Preserved .extra_usage inside the builtin branch's cache snapshot so a freshly fetched value is not clobbered on the next tick.
• Cleaned up the empty stampede-sentinel cache file on failed fetch.
• PowerShell helper is now try/catch-wrapped since the builtin branch has no outer catch.

README.md

• Install section leads with the clone-and-let-Claude-configure flow; paste-install kept as an alternative.
• Added an "Updating" section showing the single git pull path.
• Cache paths updated to reflect the per-config-dir hash suffix.

INSTALL.md (new)

• OS detection, clone target, per-shell settings.json snippets, restart, update, uninstall, requirements.
• Structured to be executed top-to-bottom by Claude Code when a user asks "clone this repo and use it as my status bar".

How to test

Fix verification (macOS / Linux)

1. Wipe the usage cache: rm -f /tmp/claude/statusline-usage-cache-*.json
2. Seed a cache entry with extra_usage.is_enabled=true to simulate a Pro/Max account with extra credits enabled:

   HASH=$(echo -n "$HOME/.claude" | shasum -a 256 | cut -c1-8)
   echo '{"five_hour":{"utilization":50,"resets_at":"2026-04-30T20:00:00Z"},"seven_day":{"utilization":30,"resets_at":"2026-05-07T20:00:00Z"},"extra_usage":{"is_enabled":true,"utilization":25,"used_credits":1234,"monthly_limit":10000}}' > "/tmp/claude/statusline-usage-cache-${HASH}.json"

3. Run the script with stdin that includes rate_limits (the builtin path):

   echo '{"model":{"display_name":"Claude"},"rate_limits":{"five_hour":{"used_percentage":42,"resets_at":1777777777},"seven_day":{"used_percentage":18,"resets_at":1778888888}}}' | bash statusline.sh

4. Expect a trailing | extra $12.34/$100.00 segment. Before this PR, the segment was silently dropped.

Empty-sentinel cleanup

1. rm -f /tmp/claude/statusline-usage-cache-*.json
2. Run with an invalid token: CLAUDE_CODE_OAUTH_TOKEN=nope echo '{"model":{"display_name":"C"}}' | bash statusline.sh
3. Cache file should not exist afterwards (so the next invocation can retry immediately).

Install docs

• Open INSTALL.md and confirm the settings.json snippets match the shell the user is running Claude Code in.
• Dry-run the clone command with --dry-run style check: git ls-remote https://github.com/daniel3303/ClaudeCodeStatusLine should respond.

Notes

• Users on Pro/Max without extra usage enabled will now make one call to /api/oauth/usage per minute per CLAUDE_CONFIG_DIR (previously zero). The endpoint is designed for this cadence and the cache is shared across tmux panes via mtime-based stampede locking.
• VERSION constant left at 1.3.0 — bump and tag a release separately when you want users to see the update-available banner.