Add AGENTS.md with Cursor Cloud dev environment instructions

[mikkelchokolate]

What

Sets up and documents the Cloud Agent development environment for Veil, and adds an AGENTS.md with a ## Cursor Cloud specific instructions section covering the toolchain, build/lint/test/run commands, and non-obvious gotchas.

This PR only adds AGENTS.md — no application code is changed. The environment update script is configured separately (via the environment settings), not in the repo.

Environment setup performed

• Toolchain: Go 1.27.0 (matches go.mod); Node 26.7.0 + pnpm 11.22.0 (matches web/package.json packageManager, Dockerfile, and scripts/ci/versions.sh). Node 26.7.0 installed via nvm (the runner injects a different node at the front of PATH, so it must be activated for frontend commands — documented in AGENTS.md).
• Built caddy v2.11.4 with the klzgrad/forwardproxy module into /usr/local/bin/caddy, which the internal/protocols and internal/protocols/naiveproxy Go tests require (they probe caddy list-modules --json).
• Update script (dependency refresh only): go mod download, then corepack pnpm install --frozen-lockfile in web/.

Verification

Area	Command	Result
Frontend deps	corepack pnpm install --frozen-lockfile	OK
Frontend build	corepack pnpm build (→ web/dist)	OK
Frontend typecheck	corepack pnpm typecheck	OK
Frontend lint	corepack pnpm check (Biome)	OK, no warnings
Frontend unit tests	corepack pnpm test (Vitest jsdom)	73 passed
API client drift	corepack pnpm gen + git diff	No drift
i18n	corepack pnpm i18n:check	CLEAN
Backend build	go build -o bin/veil ./cmd/veil (embeds web/dist)	OK
Backend vet	go vet ./...	OK
Backend tests	go test ./...	All pass (incl. internal/protocols* once caddy is present)
Run app	veil serve + privileged helper	First-run admin created, login, authenticated Panel

Hello-world demo

Ran the Panel end-to-end (privileged helper as root + veil serve as the veil user), created the initial administrator via the first-run setup, logged in, and used the authenticated Panel (Overview + Users showing the admin user).

veil_panel_clean_first_run_and_login_demo.mp4

Authenticated Veil Panel Overview dashboard

Veil Panel Users page listing the admin user

Note: the demo surfaced minor post-login/logout routing quirks in the app UI. These are existing application behaviors, unrelated to environment setup, and are out of scope for this PR.

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