fix: silence stderr on best-effort package-manager probes

[huangguang1999]

Problem

On a machine where bun is on PATH but its global directory was never initialized (the user never ran bun add -g — e.g. they installed ccstatusline via npm install -g), launching the config TUI prints bun's error straight into the terminal:

error: No package.json was found for directory "C:\Users\alice\.bun\install\global"

(Reported downstream in a localized fork: huangguang1999/ccstatusline-zh#39, with screenshots.)

Root cause

Installation inspection probes bun pm bin -g (plus where/which, npm prefix -g, npm root -g) through execFileSync without an stdio option. Node's default inherits the child's stderr into the parent terminal, so even though the thrown error is caught (catch { return null; }), the stderr text has already leaked. On Windows, where similarly writes INFO: Could not find files... to stderr when nothing matches.

Other probes in the codebase already suppress stderr (e.g. usage-fetch.ts uses stdio: ['pipe', 'pipe', 'ignore']) — these calls just missed the pattern.

Fix

Add stdio: ['ignore', 'pipe', 'ignore'] to the best-effort probe calls (stdout stays piped for the return value). No behavior change: these functions already treat any failure as "not found".

Tests

• New test asserting every probe execFileSync call carries stderr suppression (regression guard).
• New test simulating the exact bun error and asserting graceful "not found" degradation.
• bun run lint clean, bun test 1630 tests green.

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