chore: CLAUDE.md: restrict build parallelism

[Kha]

No description provided.

[mathlib-lean-pr-testing]

Mathlib CI status (docs):

• ❗ Batteries/Mathlib CI will not be attempted unless your PR branches off the nightly-with-mathlib branch. Try git rebase 78df48bdf4095e4a3b1dfda416bb3318aa0abfd7 --onto 309f44d00757482e1c5a2b2427fdca818a24615a. You can force Mathlib CI using the force-mathlib-ci label. (2026-02-20 18:22:10)
• ❗ Batteries/Mathlib CI will not be attempted unless your PR branches off the nightly-with-mathlib branch. Try git rebase 78df48bdf4095e4a3b1dfda416bb3318aa0abfd7 --onto 527a07b3adbc0c087f75899a10278f4c19a83e31. You can force Mathlib CI using the force-mathlib-ci label. (2026-02-21 14:01:09)

[leanprover-bot]

Reference manual CI status:

• ❗ Reference manual CI will not be attempted unless your PR branches off the nightly-with-manual branch. Try git rebase 78df48bdf4095e4a3b1dfda416bb3318aa0abfd7 --onto 5c7a508e21c5ebc710b0dfe65648d5beba4e34e0. You can force reference manual CI using the force-manual-ci label. (2026-02-20 18:22:12)
• ❗ Reference manual CI will not be attempted unless your PR branches off the nightly-with-manual branch. Try git rebase 78df48bdf4095e4a3b1dfda416bb3318aa0abfd7 --onto 8038a8b8904f89ad9542c8eda11379f8f006eab1. You can force reference manual CI using the force-manual-ci label. (2026-02-21 14:01:11)