chore: enforce Go formatting with make fmt and pre-push hook

[matt-dz]

Summary

• Updated AGENTS.md Code Style section to emphasize that make fmt must always be run after making any edits
• Added fmt target to the Makefile (go fmt ./...)
• Applied go fmt formatting fixes to allowedsymbols/symbols_internal.go
• Added a hooks/pre-push hook that blocks pushes containing unformatted Go files

How the pre-push hook works

The hook leverages the existing Datadog global git hooks infrastructure (/usr/local/dd/global_hooks/), which is configured for all Datadog engineers via core.hooksPath. When you run git push:

1. Git invokes the global pre-push hook
2. The global hook delegates to run-local-hooks, which automatically discovers and executes hooks in the repo's hooks/ directory
3. Our hooks/pre-push runs gofmt -l . to find any unformatted Go files
4. If any are found, the push is blocked with a message listing the offending files and directing the developer to run make fmt

No per-repo setup is required — it works out of the box for anyone with the standard Datadog git hooks configured.

Test plan

• Verify make fmt runs successfully
• Verify hook passes when all files are formatted
• Verify hook blocks push and lists unformatted files when formatting is off
• Confirm CI passes

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