docs: benchmark & CI recommendations in AGENTS.md

[azrod]

Summary\n\nAdd benchmark and CI recommendations to AGENTS.md.\n\n## Why\n\nThe repository ships exhaustive benchmarks that can be memory- and CPU-intensive (notably which may allocate ~1 GiB). Running these on small CI runners causes OOMs and long runtimes.\n\n## Changes\n\n- Add a Benchmark & CI Recommendations section to explaining safe commands for CI and how to run the full benchmark intentionally.\n- Reference README update clarifying units and bench guidance.\n\n## Testing\n\n- === RUN TestEncryptDecrypt

--- PASS: TestEncryptDecrypt (0.22s)
=== RUN TestEncryptDecryptRaw
--- PASS: TestEncryptDecryptRaw (1.03s)
=== RUN TestDifferentPasswordsFail
--- PASS: TestDifferentPasswordsFail (0.44s)
=== RUN TestDifferentParamsFail
--- PASS: TestDifferentParamsFail (0.62s)
=== RUN TestBase64InputOutput
--- PASS: TestBase64InputOutput (0.06s)
=== RUN TestEmptyPlaintext
--- PASS: TestEmptyPlaintext (0.22s)
=== RUN TestTruncatedCiphertext
--- PASS: TestTruncatedCiphertext (0.20s)
=== RUN TestTamperedCiphertext
--- PASS: TestTamperedCiphertext (0.20s)
=== RUN TestConcurrentEncrypt
--- PASS: TestConcurrentEncrypt (0.74s)
PASS
ok github.com/azrod/cryptio (cached) and === RUN TestEncryptDecrypt
--- PASS: TestEncryptDecrypt (1.94s)
=== RUN TestEncryptDecryptRaw
--- PASS: TestEncryptDecryptRaw (9.60s)
=== RUN TestDifferentPasswordsFail
--- PASS: TestDifferentPasswordsFail (4.81s)
=== RUN TestDifferentParamsFail
--- PASS: TestDifferentParamsFail (6.21s)
=== RUN TestBase64InputOutput
--- PASS: TestBase64InputOutput (0.70s)
=== RUN TestEmptyPlaintext
--- PASS: TestEmptyPlaintext (1.88s)
=== RUN TestTruncatedCiphertext
--- PASS: TestTruncatedCiphertext (2.72s)
=== RUN TestTamperedCiphertext
--- PASS: TestTamperedCiphertext (1.96s)
=== RUN TestConcurrentEncrypt
--- PASS: TestConcurrentEncrypt (22.24s)
PASS
ok github.com/azrod/cryptio (cached) ran locally. All tests passed.\n\nPlease review and merge if acceptable.