docs: add Cursor Cloud dev environment setup instructions to AGENTS.md

[fabio-silva]

Disclaimer: this PR was fully written by AI

What

Adds a ## Cursor Cloud specific instructions section to AGENTS.md capturing the non-obvious, durable steps for bringing up the PMM development environment in a Cloud Agent VM. No product code is changed.

Why

Setting up PMM for development surfaced several gotchas that aren't documented elsewhere and would trip up future agents:

• The dev environment is a single "fat" Docker container (perconalab/pmm-server:3-dev-container) running all server components under supervisord; binaries are rebuilt on the host bind mount and hot-swapped.
• Use .env.dev.example (dev image + toolchain), not .env.example (release image).
• The run-* / init / check / build-ui targets must run as root in the container (make env-root TARGET=...) — the default pmm user can't write to /usr/sbin or the root-owned Go module cache volume.
• Git "dubious ownership" on the bind mount must be cleared inside the container before building.
• go.mod pins a newer Go than the image ships; Go auto-downloads the toolchain (works as root).
• golangci-lint is not preinstalled (make env-root TARGET=init).
• Node/Yarn are missing from the dev image and must be installed in the container for UI dev.

Verification

Brought the full environment up and exercised it end-to-end:

• make env-up — container healthy; all supervisord services running.
• Rebuilt pmm-managed from source and hot-swapped it (run-managed-ci); /v1/server/version reports the git commit as managed.full_version.
• make check (buf lint + golangci-lint + go-sumtype): 0 issues.
• make test-common: all packages ok.
• build-ui: UI built with yarn and deployed into Grafana.
• Hello-world: logged into the UI (admin/admin) and added a monitored PostgreSQL service hello-world-postgres; its postgres_exporter is AGENT_STATUS_RUNNING.

pmm_add_postgresql_service.mp4

Adding a monitored PostgreSQL service through the freshly-built backend and UI.

Inventory showing hello-world-postgres
PMM home dashboard

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[it-percona-cla]

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 43.60%. Comparing base (31318c7) to head (acdb4ea).
⚠️ Report is 10 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #5617   +/-   ##
=======================================
  Coverage   43.59%   43.60%           
=======================================
  Files         415      415           
  Lines       43134    43133    -1     
=======================================
+ Hits        18804    18806    +2     
+ Misses      22454    22452    -2     
+ Partials     1876     1875    -1     

Flag	Coverage Δ
admin	34.78% <ø> (ø)
agent	49.04% <ø> (ø)
managed	42.99% <ø> (+<0.01%)	⬆️
vmproxy	72.22% <ø> (ø)

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