docs(blog): add agentOS execution API for JavaScript and Python post

[NathanFlurry]

• Adds the agentOS execution API launch post for JavaScript and Python
• Covers evaluate, execute, inputs, retained contexts, type checking, npm and pip installs, background processes and web servers, and the Linux-compatible environment
• Uses Jacques Linard's "Still Life with Shells and Coral" (public domain) as the hero, uploaded to R2
• Validated with a full website build; post renders at /changelog/2026-07-28-introducing-agentos-execution-api-for-javascript-and-python/

[claude]

Review

This PR adds a single blog/changelog post (page.mdx) announcing the agentOS execution API for JavaScript and Python. No application code changes, so the review focuses on content accuracy and consistency of the code samples.

Frontmatter / conventions - all good:

• author, category, published, image: { file: "image.png?v=2" } all match the schema in website/src/content.config.ts and the .claude/reference/content-frontmatter.md conventions.
• agentOS spelling and rivet.dev / agentos-sdk.dev domains are used consistently per the root CLAUDE.md terminology rules.
• @nocheck usage is appropriate since @rivet-dev/agentos's execution API doesn't exist in this repo yet (matches the documented exception in website/CLAUDE.md).

Issues found in the code samples:

1. Networking example won't work as written - in "A full Linux-compatible environment underneath" (both the JavaScript and Python tabs), the snippet fetches http://localhost:8000/ but this block creates a brand new AgentOs.create(...) and never spawns anything listening on port 8000 within it (unlike the earlier "Background processes and web servers" section, which explicitly spawns a server before hitting it). A reader copy-pasting this exact block will get a connection failure instead of the commented // 200. Either spawn a quick listener in this snippet too, or note that it continues from the previous section's server.

2. Minor API-surface inconsistency in "Keep state between calls": context creation is a top-level shorthand (await runtime.createContext("analysis")), but cleanup is namespaced (await runtime.contexts.delete("analysis")). If that split is intentional (common op gets a shorthand, less-common ops live under .contexts), might be worth a one-line justification since it reads as an inconsistent surface otherwise.

3. Minor naming asymmetry in "Install packages programmatically": npm install is namespaced (runtime.javascript.npm.install(...)), while pip install is not (runtime.python.install(...) rather than something like runtime.python.pip.install(...)). Likely intentional since Python doesn't have competing package managers the way JS does, but worth double-checking against the real SDK surface before this ships.

Not verified: I couldn't reach the network from this sandbox to confirm the R2-hosted hero image (https://assets.rivet.dev/website/blog/2026-07-28-introducing-agentos-execution-api-for-javascript-and-python/image.png?v=2) actually resolves - worth a quick manual check before merge if that wasn't already done as part of the "full website build" validation mentioned in the PR description.

Overall this is a well-structured, on-convention changelog post; the flagged items are all sample-code polish rather than blockers.