fix(core): remove Node-only code to restore browser compat [closes #281]

[EmersonBraun]

Summary

P0 for Phase 0 launch. Restores browser compatibility of @agentskit/core, fulfilling Manifesto principle 1 ("core works in any environment — Node, Deno, edge, browser").

Closes #281. Unblocks the 5 browser-side E2E tests currently skipped in #282.

The problem

Since v0.x, packages/core/src/config.ts had:

import { readFile } from 'node:fs/promises'

at the top of the file. Browser bundlers externalize node:* but the import still runs at module load time, crashing every browser consumer with:

Module "fs/promises" has been externalized for browser compatibility. Cannot access "fs/promises.readFile" in client code.

Result: apps/example-react never mounted. <div id="root"> stayed empty. The 5 skipped tests in #282 were detecting this.

The fix

Relocate two Node-only helpers out of @agentskit/core:

Before	After
createFileMemory (core)	fileChatMemory (@agentskit/memory)
loadConfig + AgentsKitConfig + LoadConfigOptions (core)	same exports, moved to @agentskit/cli

What stays in core (all universal):

• createInMemoryMemory — pure JS
• createLocalStorageMemory — already guards typeof localStorage

Breaking changes

Declared in .changeset/core-browser-compat.md:

- import { createFileMemory } from '@agentskit/core'
+ import { fileChatMemory } from '@agentskit/memory'

- import { loadConfig } from '@agentskit/core'
+ import { loadConfig } from '@agentskit/cli'

Bumps:

• @agentskit/core major (exports removed)
• @agentskit/memory minor (fileChatMemory added)
• @agentskit/cli minor (loadConfig now public)
• @agentskit/react minor (dropped re-export)
• @agentskit/ink minor (dropped re-export)

The removed re-exports in @agentskit/react and @agentskit/ink were always Node-only — browser consumers got a runtime error from them anyway, so this is making the shape honest.

Verification

• ✅ grep fs/promises packages/core/dist/*.js → zero matches
• ✅ grep "from 'node:" packages/core/dist/*.js → zero matches
• ✅ pnpm --filter "./packages/*" build → all green
• ✅ Manual browser check on apps/example-react: root now mounts with the full chat UI, no pageerrors
• ✅ pnpm test → only the pre-existing ink@7 failure (fixed in foundation: manifesto, origin, governance templates, and Phase 0 planning docs #265), unrelated to this change

Test plan

• Core dist is Node-builtin-free
• Example-react mounts in a real browser
• Tests relocated to the right packages
• Changeset describes the migration
• Reviewer: CI confirms bundle size gate still passes (core should be slightly smaller now)
• Follow-up PR: re-enable the 5 skipped E2E tests in test(e2e): Playwright coverage for all 4 example apps [closes #251] #282 and adjust placeholder selector

Refs #281 #282 #265 #211