feat(tasks): add skill bundle artifact contracts and metadata types

[tatoalo]

Problem

for cloud tasks, local skill invocations only sent the slash command text. User-local and repo-local skills were available in the local editor but not in the cloud sandbox

Changes

• let's bundle uploadable local skills into zip artifacts with SHA-256 metadata and send them through the existing cloud artifact upload flow.

• install skill_bundle artifacts in the agent server before prompt delivery, inject the bundled skill instructions for the invoked turn, and suppress the raw slash command from the model-facing prompt so local skills behave like actual skills rather than unsupported commands

• refresh slash command metadata when a follow-up command is submitted, so newly created local skills can be selected and invoked without starting a brand-new task.

closes https://github.com/PostHog/code/issues/2260

[tatoalo]

• feat(tasks): surface local skills in the composer and upload on send #2926
• feat(tasks): route local skill tags through the cloud prompt pipeline #2925
• feat(tasks): install and apply skill bundle artifacts in the sandbox #2924
• feat(tasks): bundle local skills into sha256-verified zip artifacts #2923
• feat(tasks): add skill bundle artifact contracts and metadata types #2922 👈 (View in Graphite)
• main

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[github-actions]

React Doctor found no issues in the changed files. 🎉

_{Reviewed by React Doctor for commit b44c3b4.}

[greptile-apps]

_{Reviews (1): Last reviewed commit: "feat(tasks): add skill bundle artifact c..." | Re-trigger Greptile}

[greptile-apps]

_{Reviews (2): Last reviewed commit: "feat(tasks): add skill bundle artifact c..." | Re-trigger Greptile}