fix(hooks): inject condensed instructions on SubagentStart

[arimu1]

Summary

• SubagentStart previously injected the full mode-filtered SKILL.md via getPonytailInstructions (~5k+ chars).
• Subagents now receive getSubagentInstructions() — the condensed getFallbackInstructions path (independent modes keep their short pointer string).
• Keeps Task-worker context smaller while preserving the same lazy-senior ruleset.

Fixes #597

Test plan

• Node load of ponytail-instructions / compare lengths: condensed < full for mode full
• Install branch locally, spawn a Task subagent with ponytail active, confirm injected context is the condensed banner rather than full SKILL.md

Made with Cursor

[huda-code]

Hi @arimu1, I added automated test coverage for the requirements discussed in #597.

The test commit is 8110543, and the full test suite passes locally:

• Root suite: 82 passed, 0 failed
• Pi extension: 23 passed, 0 failed
• Ponytail MCP: 3 passed, 0 failed

The tests verify the condensed SubagentStart payload, independent modes, matched and nonmatching agent behavior, fail-open cases, and the Claude, Codex, and Qoder paths.

Could you please cherry-pick the commit into this PR?

git cherry-pick 8110543

I closed #622 because it was accidentally opened against main and duplicated this implementation PR.

[arimu1]

Thanks @huda-code — cherry-picked 8110543 (test(hooks): cover condensed SubagentStart payload) into this branch. Root suite + pi-extension + ponytail-mcp all green locally.