Add Eve addon guide

[broomva]

Description

Adds docs/content/docs/addons/eve.mdx — an addon guide for integrating Eve (Vercel's open-source durable-agent framework) as a workspace app in a next-forge monorepo, following the existing addon shape (c15t, Trunk).

The guide covers: scaffolding apps/agent with eve init (including the workspace-root changes it makes), a tool that reuses @repo/database's generated Prisma client and validated env keys with graceful degradation when DATABASE_URL is absent, running and exercising the HTTP channel, evals, client wiring via eve/react, and deploying as a separate Vercel project.

Related Issues

Closes #754

Checklist

• My code follows the code style of this project. (frontmatter, <Tip>/<Steps>, imperative voice, structure mirrored from sibling addon pages)
• I have performed a self-review of my code. (plus two rounds of adversarial cross-model review; details below)
• I have commented my code, particularly in hard-to-understand areas.
• I have updated the documentation, if necessary. (this PR is documentation)
• I have added tests that prove my fix is effective or my feature works. (N/A — docs-only; see verification notes)
• New and existing tests pass locally with my changes. (docs app compiles the page without content errors)

Additional Notes

Every command and caveat in the guide was verified on a fresh next-forge@6.0.2 scaffold with eve@0.20.0 (scaffold initialized with the pnpm option; command forms in the guide follow this repo's Bun convention):

Verified	Result
npx eve@latest init . inside apps/agent	✓ generates the 3 agent files; root engines.node rewritten >=18 → 24.x; pnpm hygiene block appended
eve dev boot + GET /eve/v1/health	✓ {"ok":true,"status":"ready"}
Full durable session turn with tool call	✓ NDJSON stream: actions.requested → action.result → assistant reply; replayable
Tool importing @repo/database root entry	✗ server-only guard crashes Eve's Nitro runtime → guide documents the generated/client + keys subpath imports
Dynamic import() inside a tool	✗ fails Eve's single-module bundling → guide mandates static imports
Graceful degradation without DATABASE_URL	✓ tool returns disabled: true; agent explains which env var enables it
eve build	✓ Nitro production output
tsc --noEmit with moduleResolution: bundler	✓
eve eval --strict (the guide's smoke eval)	✓ 3/3 gates

The page was additionally put through two rounds of adversarial review against the eve 0.20.0 source and this repo's conventions (initial findings included pnpm-flavored commands, a snippet that would clobber eve init's deps, a missing prisma generate precondition, and eve init's patch-wins semantics silently un-pinning the exact version — all fixed in the current revision).

Eve is in beta; the guide pins eve@0.20.0 (scaffolder and dependency) and flags the churn explicitly.

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