fix: build mastra package export

[mojobeeping]

Summary

The published @trysiren/agent-toolkit@0.2.0 package declares a ./mastra subpath export, but the published tarball does not include the target files under dist/mastra/*.

That makes this documented package entrypoint fail at runtime with ERR_MODULE_NOT_FOUND.

This PR adds src/mastra/index.ts to the tsup entry list so the dist/mastra bundle and declarations are generated, and externalizes the Mastra peer dependency the same way the OpenAI, LangChain, and AI SDK peer dependencies are handled.

Reproduction

tmpdir=$(mktemp -d)
cd "$tmpdir"
npm init -y >/dev/null
npm install --ignore-scripts @trysiren/agent-toolkit@0.2.0
node --input-type=module -e "import('@trysiren/agent-toolkit/mastra')"

Observed result from the published package:

Error [ERR_MODULE_NOT_FOUND]: Cannot find module .../node_modules/@trysiren/agent-toolkit/dist/mastra/index.js

The package metadata points exports["./mastra"] at:

{
  "import": "./dist/mastra/index.js",
  "require": "./dist/mastra/index.js",
  "types": "./dist/mastra/index.d.ts"
}

But the published tarball has no dist/mastra files.

Validation

Ran from typescript/:

npm run build
npm run type-check
npm test -- --runInBand
git diff --check
NPM_CONFIG_CACHE=/tmp/mojobeep-npm-cache npm pack --json --ignore-scripts

The packed tarball now includes:

package/dist/mastra/index.js
package/dist/mastra/index.mjs
package/dist/mastra/index.d.ts
package/dist/mastra/index.d.mts

I also extracted the packed tarball into a clean temp directory and verified both loaders resolve the fixed entrypoint:

PATCHED_TRYSIREN_MASTRA_REQUIRE_OK SirenAgentToolkit,SirenTool
PATCHED_TRYSIREN_MASTRA_IMPORT_OK SirenAgentToolkit,SirenTool
PATCHED_TRYSIREN_MASTRA_FILES_OK

One unrelated check still fails before reaching changed files:

npm run lint
TypeError: Key "rules": Key "@typescript-eslint/prefer-const": Could not find "prefer-const" in plugin "@typescript-eslint". Did you mean "@/prefer-const"?