fix(hooks): silence off-mode SessionStart

[connerohnesorge]

Summary

• Emit no SessionStart stdout when the configured mode is off.
• Preserve stale mode-state cleanup.
• Cover native Claude, Codex, and Copilot with one regression check.

Why

The native Claude off branch currently passes OK to the output formatter. This creates model-visible startup context even though off mode promises no output. Codex and Copilot also format the empty context into structured stdout.

Skipping the formatter in the shared off branch makes all three harnesses silent while leaving active-mode output unchanged.

Verification

• The new check fails on unmodified main with actual Claude stdout OK.
• node --test tests/hooks.test.js
• Remaining root tests excluding the baseline-broken pandas correctness probe: 68 passed.
• npm test --prefix pi-extension: 23 passed.
• npm test --prefix ponytail-mcp: 3 passed.

The full root command has one existing local failure in csv: correct pandas one-liner passes. The same failure reproduces on unmodified main.