feat(platform): add Extension runtime architecture, debugging skills

[MajorLift]

Adds a platform domain: the extension's runtime architecture and the failure modes specific to it.

The MV3 service-worker lifecycle, extension architecture, and runtime-error triage apply to metamask-extension and nowhere else — they have no counterpart in Mobile or Core, which is why they need a domain rather than a corner of coding.

Skills

• extension-errors-debugging — triage an extension runtime error to root cause. Ships a metamask-extension overlay.
• extension-lifecycle-decoupling — decouple work from the MV3 service-worker lifecycle, so a suspended worker doesn't strand it.

Knowledge

• extension-architecture — the process/context model the two skills reason over.
• mv3-service-worker — lifecycle, suspension, and what survives it.

Notes

• New domain with no CODEOWNERS entry — /domains/platform/ falls to the * catch-all. Tell me which team should own it and I'll add the line.
• No CHANGELOG entry. Per CONTRIBUTING, that file tracks consumer-facing changes to the @metamask/skills CLI package; skill-only PRs don't add one.
• Both skills are experimental. Commits GPG-signed.

[MajorLift]

Context budget

What this PR costs an agent, measured from an install rather than read from the diff. Three tiers, and only the first is unavoidable.

Skill	Frontmatter	Selected	+ refs & knowledge
extension-errors-debugging	84 chars	~1,355 tok	~1,355 tok
extension-lifecycle-decoupling	90 chars	~1,178 tok	~1,178 tok

Frontmatter is the only tier paid unconditionally — every agent loads it on every run once the skill is installed, used or not, because it is what the agent reads to decide relevance. The 28 skills across the eleven open skill PRs sit at a median of ~1,716 tokens selected and ~1,860 with references followed. All are within the 1,536-character description budget.

Selected is paid only when the agent picks the skill. + refs & knowledge is the ceiling if every bundled reference is then read; it is a worst case, not an expectation.

Method

tools/install --repo metamask-extension --maturity experimental against this branch at 45f78e127, measured per installed skill directory. Repo overlays are merged into the emitted SKILL.md, so they land in the selected tier rather than being missed by a source-byte count. Token figures are bytes/4 — a proxy for scale, not accounting.

These figures are pinned to the commit above and drift on every push; #96 tracks automating them.