Need a substrate developer troubleshooting guide

## Summary

Substrate has good documentation for the happy path and none for what happens when it breaks.

- `README.md` — quickstart, both kind and GKE. Assumes everything works.
- `CONTRIBUTING.md` — process, PR sizing, root-gated tests, copyright headers.
- `AGENTS.md` — layout, build/test command reference.
- `docs/dev/code-layout.md` — where code goes.

So a contributor whose image didn't update, whose lint output is silently truncated, or whose e2e run fails on a laptop has nothing to consult. Substrate is a harder-than-average project to debug: gVisor and micro-VM sandboxes, snapshot/restore, a custom network path, a control plane deliberately outside the Kubernetes control plane, and two very different deployment targets — and the traps are mostly **silent or misleading**, not loud.

This issue asks for `docs/dev/troubleshooting.md`, organized by where you are in the dev loop.

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## Scope

Contributors working *on* Substrate. Distinct from the two other guides: the actor developer (building workloads on Substrate) and the operator (running a fleet).

A telemetry-specific troubleshooting draft already exists as a sibling issue; it should either become a section here or stay a linked companion. Recommend companion — it is deep enough to stand alone, and this guide should link to it rather than absorb it.