Fix quickstart configure_factory to discover non-Go repos

[stempeck]

Summary

• Remove the redundant && [ -f "$d/go.mod" ] clause from configure_factory() in quickstart.sh so workspace discovery selects any cloned repo by its .git directory alone, making it tech-stack-agnostic
• Update the discovery comment and error message to no longer reference go.mod
• Add TestQuickstartConfigureFactoryDiscovery tests locking in stack-agnostic discovery, the .git filter, the updated error message, and the preserved install_af source go.mod check

Problem

configure_factory() required both .git and go.mod to identify the target repo under $WORKSPACE_DIR. The go.mod check provided zero disambiguation (the only competing directory, aftmp, has no .git and was already excluded), but broke setup for any non-Go customer repo.

Fix

Minimal clause removal: drop && [ -f "$d/go.mod" ] from the discovery loop, retain the [ -d "$d/.git" ] filter. The install_af AF-source go.mod check is untouched.

Fixes #55

Test plan

• TestQuickstartConfigureFactoryDiscovery verifies configure_factory has no go.mod reference
• TestQuickstartConfigureFactoryDiscovery verifies .git filter is preserved
• TestQuickstartConfigureFactoryDiscovery verifies error message says "No git repository" without go.mod
• TestQuickstartConfigureFactoryDiscovery verifies install_af still checks $SCRIPT_DIR/go.mod for agentfactory

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