docs(examples): unblock 3 up canaries (compose layout, env-var, stderr capture)

[pofallon]

Three small canary fixture fixes found while running the up suite post-#120.

Summary

• `compose-basic` and `compose-profiles`: rename `devcontainer.json` → `.devcontainer.json` so deacon's spec-aligned discovery finds them (same pattern as examples/read-configuration: 8 canaries unrunnable (non-standard config layout) + legacy-normalization README is factually wrong #104, examples/exec: 8 canaries unrunnable (non-standard config layout) #110, docs(examples): fix python heredoc stdin collision in 13 canaries (#115) #116).
• `compose-profiles/exec.sh`: `run COMPOSE_PROFILES=… cmd` doesn't propagate the env var because `run` treats it as a positional arg. Use `env COMPOSE_PROFILES=… cmd` so the variable actually reaches the child.
• `gpu-modes/exec.sh`: redirect deacon's stderr to a temp log rather than capturing `2>&1` inside `$( … )`. Post-fix(up): route lifecycle command stdout to stderr (#113) #114 the result JSON lives only on stdout; the prior `2>&1` capture muddled it with the warning logs and broke the python parse.

Test plan

• All 3 canaries run end-to-end with exit 0:
  • `examples/up/compose-basic/exec.sh`
  • `examples/up/compose-profiles/exec.sh` (5 profile scenarios)
  • `examples/up/gpu-modes/exec.sh` (none/detect/all)

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