fix(cli): avoid os.cpus during startup prompt

[roboomp]

Repro

A focused regression test spies on node:os.cpus while building a minimal system prompt: bun test /tmp/system-prompt-os-cpus-repro.test.ts. Before the fix it failed with Expected number of calls: 0 and Received number of calls: 1, proving buildSystemPrompt() synchronously reached os.cpus() on the startup prompt path.

Cause

packages/coding-agent/src/system-prompt.ts built the workstation block via getEnvironmentInfo(), which read os.cpus()[0]?.model; on Linux/Bun this can trigger per-logical-CPU sysfs frequency reads before the TUI becomes usable.

Fix

• Added async Linux CPU model discovery that parses the first model name from /proc/cpuinfo and returns no CPU line on platforms without that cheap source.
• Moved CPU discovery into the existing bounded system-prompt preparation fan-out so it shares timeout/fallback handling with other startup probes.
• Added a regression test asserting system prompt construction does not call os.cpus().
• Added the coding-agent changelog entry for Bug: omp startup blocks for seconds on many-core Linux hosts due to os.cpus() in system prompt #4712.

Verification

bun test packages/coding-agent/src/system-prompt.test.ts passed: 5 tests, 12 expectations. bun test /tmp/system-prompt-os-cpus-repro.test.ts passed after the fix. bun check passed for all workspace packages. Fixes #4712

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