fix: discover nested agent context plans

[luohui1]

Description

Closes #3024 by making the agent-context update scripts discover the newest plan.md anywhere under specs/, including nested/scoped feature directories such as specs/scope/feature/plan.md.

Bash and PowerShell script behavior stays in parity, and the extension command documentation now describes recursive plan discovery.

Split out from #3053 after maintainer feedback to keep each bug fix in its own PR.

Testing

• Tested locally with uv run specify --help
• Ran existing tests with uv sync && uv run pytest
• Tested with a sample project (if applicable)

Additional validation run locally:

• uv run --extra test pytest tests/extensions/test_extension_agent_context.py -q - 35 passed, 1 skipped
• uvx ruff check tests/extensions/test_extension_agent_context.py - passed
• git diff --check - passed

Manual test results

Agent: Codex (GPT-5) | OS/Shell: Windows 11 / PowerShell 7.5.4

Command tested	Notes
.specify/extensions/agent-context/scripts/powershell/update-agent-context.ps1	Covered by regression test with nested specs/scope/feature/plan.md.

AI Disclosure

• I did not use AI assistance for this contribution
• I did use AI assistance (describe below)

This PR was implemented by Codex (GPT-5) acting as an AI coding agent on behalf of @luohui1. The agent read the repository contribution/security/code-of-conduct docs and issue templates, inspected the relevant code paths, implemented the changes, ran the validation listed above, and prepared this PR description.