feat(edge-worker): cloud-runtime package-install system prompt addendum (CYPACK-1266)

[cyrusagent]

Summary

Implements CYPACK-1266: on Cyrus-managed cloud runtimes, the agent now knows that missing system packages must be added through the dashboard rather than installed ad-hoc.

CYHOST-994 introduces a way for users to add system-wide apt/npm packages to their VPS. Since Cyrus has no intrinsic way of knowing it's running on a managed cloud runtime, this PR gates the behavior behind a new env var, CYRUS_CLOUD_RUNTIME. When it's truthy, a system-prompt addendum tells the agent: if a package is missing, don't try to sudo apt install / npm install -g (won't persist, may fail on perms) — instead direct the user to https://app.atcyrus.com/settings/packages to add the exact package(s) needed, naming them and explaining why, and continue with whatever work isn't blocked.

Approach (SOLID)

Mirrors the existing browser-use addendum precedent exactly:

• New single-responsibility module packages/edge-worker/src/prompts/cloudRuntimePromptAddendum.ts — owns the addendum text and a pure appendCloudRuntimeAddendum() that is a no-op unless CYRUS_CLOUD_RUNTIME is truthy (1/true/yes, case/space-insensitive).
• Chained at the single composition point — RunnerConfigBuilder builds appendSystemPrompt for both chat and issue sessions; the new addendum wraps the existing appendBrowserUseAddendum(appendFailureModeAddendum(...)) chain at both sites.

Code paths considered (cold start + hot refresh)

RunnerConfigBuilder is the single point through which every runner config flows — both the chat-session and issue-session build sites are updated. The env var is read fresh each time a runner config is assembled, so it applies identically on cold starts (new session) and continuation/resume runs. No config-schema plumbing is needed since this is a runtime env var, not a config.json field — so there's no ConfigManager whitelist or path-normalization codepath to update.

Testing

• New unit test cloudRuntimePromptAddendum.test.ts (6 cases): unset/falsy → unchanged, truthy spellings, separator behavior, verbatim-when-empty, and presence of the settings link + apt/npm guidance.
• Full edge-worker suite green (676 tests).
• Monorepo build + typecheck pass (run via pre-commit hook).

Changelog

Added an ## [Unreleased] entry referencing CYPACK-1266 and this PR.