fix: surface GitHub API rate limit errors in install scripts with VPN suggestion

[devin-ai-integration]

Summary

When the GitHub API returns a rate limit error during CLI installation, the install scripts previously failed silently — the grep for browser_download_url would match nothing, DOWNLOAD_URL would be empty, and the user would either get a confusing "Could not find release" error (install-latest.sh) or a completely silent failure (install-latest-linux.sh).

This PR stores the API response in a variable first, checks it for the "API rate limit exceeded" string, and if detected:

• bin/install-latest.sh and bin/install-latest-linux.sh: Exit with a clear error message and a suggestion to turn off VPN.
• scripts/install-agent-skill.sh: Warns but continues (since skill files are already downloaded at that point; only .version resolution is skipped).

Additionally, the generic "Could not find release" fallback in both install scripts now dumps the raw API response to stderr for easier debugging of other unexpected API failures.

Review & Testing Checklist for Human

• Verify the grep pattern matches real rate-limit responses: The check uses grep -q "API rate limit exceeded" against the raw JSON body. Confirm this matches the actual GitHub API error message (e.g., {"message":"API rate limit exceeded for ...","documentation_url":"..."}). If GitHub changes the wording, the check silently becomes a no-op (falls through to old behavior).
• Test manually with a rate-limited IP: Simulate or trigger a real rate limit response and confirm the error output is clear and the script exits non-zero. Also test the happy path to confirm no regression.
• Review the agent skill script's softer handling: install-agent-skill.sh only warns on rate limit (doesn't abort) since it only affects .version resolution. Confirm this is the desired UX — a user might not notice the warning if there's lots of output.

Notes

• The VPN suggestion is the primary user-facing guidance since the reported issue originated from corporate VPN traffic sharing a single IP that hits rate limits. This won't help in all cases (e.g., CI environments), but it's the most common cause for end users.
• No automated test coverage for these shell scripts — manual testing is the only verification path.

Link to Devin session: https://app.devin.ai/sessions/865ae722ee8243c08d80b2bb8d639599
Requested by: @theFong

[devin-ai-integration]

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