docs(deploy-to-cloud-engine): proxy-canister usage for cross-subnet calls

[pietrodimarco-dfinity]

What

Adds a Cross-subnet calls via a proxy canister section to the deploy-to-cloud-engine skill, plus four pitfalls and a widened description so the skill triggers on proxy / XRC / threshold-key questions.

Why

A cloud engine runs on a CloudEngine subnet, which the protocol forbids from sending cross-subnet messages that carry cycles or are guaranteed-response. So an engine canister cannot directly call the exchange-rate canister, threshold ECDSA/Schnorr, or vetKD — all of which are cross-subnet and cycle-bearing. Agents writing engine apps hit this and need the proxy-canister workaround, which was previously undocumented in the skill.

Contents

• When a proxy is (and isn't) needed.
• Deploying + funding a proxy from the engine console (Applications → Proxy canisters; initial balance, optional auto top-up).
• Calling targets through the proxy's proxy method — full candid interface + Motoko sketch, with ProxyArgs.cycles semantics and ProxyError handling.
• Threshold-key caveat: the proxy caller-isolates key derivation, so a key/address obtained through the proxy differs from a direct management-canister call — fetch the public key and sign through the same proxy consistently.
• Pitfalls 10–13: direct cross-subnet calls, attaching cycles to the outer proxy call, running the proxy dry / funding via CLI, and expecting a direct-call key through the proxy.

Validation

• node scripts/check-project.js passes (pre-existing warnings only, unrelated skills).
• SKILL.md is 276 lines (< 500).
• skill-validator not run (not installed in my environment).

🤖 Generated with Claude Code

[github-actions]

Skill Validation Report

Validating skill: /home/runner/work/icskills/icskills/skills/deploy-to-cloud-engine

Structure

• Pass: SKILL.md found

Frontmatter

• Pass: name: "deploy-to-cloud-engine" (valid)
• Pass: description: (1010 chars)
• Pass: license: "Apache-2.0"
• Pass: compatibility: (127 chars)
• Pass: metadata: (2 entries)

Markdown

• Pass: no unclosed code fences found

Tokens

File	Tokens
SKILL.md body	4,975
Total	4,975

Content Analysis

Metric	Value
Word count	3,253
Code block ratio	0.10
Imperative ratio	0.11
Information density	0.11
Instruction specificity	0.97
Sections	13
List items	64
Code blocks	10

Contamination Analysis

Metric	Value
Contamination level	low
Contamination score	0.09
Primary language category	shell
Scope breadth	3

• Warning: Language mismatch: config (1 category differ from primary)

Result: passed

Project Checks

✓ Project checks passed for 1 skills (0 warnings)