ci: enforce bundle size budget with size-limit [closes #217]

[EmersonBraun]

Summary

Adds size-limit to enforce gzipped bundle sizes on every PR. Closes #217 and verifies Manifesto principle 1 (@agentskit/core < 10 KB gzipped).

How it works

• .size-limit.json declares budgets per package (gzipped, ESM + CJS where relevant)
• Local commands:
  • pnpm size — verify all packages
  • pnpm size:why — bundle inspector
• CI workflow .github/workflows/size.yml runs on every PR + push to main; blocks merge if any package exceeds its budget

Current measured sizes (all under budget)

Package	Limit	Actual	Headroom
@agentskit/core (ESM)	10 KB	5.22 KB	48%
@agentskit/core (CJS)	10 KB	5.30 KB	47%
@agentskit/adapters	20 KB	4.32 KB	78%
@agentskit/react	15 KB	2.58 KB	83%
@agentskit/runtime	15 KB	2.34 KB	84%
@agentskit/memory	15 KB	2.73 KB	82%
@agentskit/rag	10 KB	852 B	92%
@agentskit/tools	15 KB	2.27 KB	85%
@agentskit/skills	10 KB	5.19 KB	48%
@agentskit/observability	10 KB	2.95 KB	70%
@agentskit/eval	10 KB	556 B	95%
@agentskit/sandbox	10 KB	1.53 KB	85%
@agentskit/ink	15 KB	1.33 KB	91%
@agentskit/cli	20 KB	177 B	99%
@agentskit/templates	15 KB	2.89 KB	81%

Why @size-limit/file instead of preset-small-lib

The small-lib preset bundles via esbuild expecting browser code. Most AgentsKit packages use Node builtins (fs/promises, path, child_process) which esbuild can't resolve without platform: 'node'. Since these are libs published to npm (consumers bundle them, not us), measuring the published file gzipped is the right metric. @size-limit/file does exactly that.

Trade-offs

• No tree-shake measurement — small-lib measures "what would a user pay if they imported the whole index". File mode measures "the published file". For libs with named exports (which we encourage in CLAUDE.md), tree-shaking handles userland anyway.
• Headroom is generous — limits are set 2-4x current size. This gives breathing room for legitimate growth without becoming a meaningless rubber-stamp. Will tighten if any package consistently grows.

Test plan

• pnpm size runs locally and passes
• pnpm build && pnpm exec size-limit produces correct numbers
• Workflow YAML validates
• CI on this PR shows the size check passing
• (Future PR test) deliberately bloat a package, confirm CI blocks merge

Follow-ups

• Add size-limit comment-on-PR action (currently CLI-only)
• Add size-limit-action from andresz1/size-limit-action for delta reporting (size went up by X% in this PR)
• Tighten budgets after 2-3 sprints of stable measurement

Closes #217
Refs #211