build: replace tsup with tsdown and make the published types reachable

[dawsontoth]

One of two alternatives. This keeps declaration emit and moves off unmaintained tooling. The other is dropping --dts and staying on tsup. Pick one, not both.

Why move

tsup 8.5.1 inlines its own rollup-plugin-dts@6.1.1 compiled against typescript@5.7.3, and that vendored copy throws on TS 7 — the blocker on #145:

TypeError: Cannot read properties of undefined (reading 'useCaseSensitiveFileNames')

Nothing in our lockfile reaches that copy, so overrides can't help. And tsup is effectively dormant:

	latest	last published
tsup	8.5.1	2025-11-12 (~10 months, only dist-tag)
tsdown	0.23.0	2026-09-03

tsdown is the rolldown-based successor from the same ecosystem. The relevant difference isn't that it's newer — it's that it emits declarations through TypeScript itself:

ℹ Emit types with typescript@7.0.2

rather than through a pinned copy of rollup-plugin-dts. That's the structural reason it isn't exposed to this class of breakage.

Bonus fix: the types were unreachable

Worth knowing regardless of which branch you take — the declarations we already build can't be resolved by anyone:

// before
"exports": { ".": "./dist/index.js" }   // no "types" condition, no "types" field

This PR adds both, so the emit we're paying for is actually usable:

"types": "./dist/index.d.ts",
"exports": { ".": { "types": "./dist/index.d.ts", "default": "./dist/index.js" } }

Config notes

• outExtensions pins .js. tsdown defaults to .mjs, which would break bin (./dist/cli.js) and exports (./dist/index.js). Caught this the first time through.
• dist/ gains .map files that tsup didn't emit. tsdown forces sourcemaps on whenever tsconfig sets declarationMap, which ours does — a sourcemap: false in the config does not override it. They aren't published: files already excludes dist/**/*.map, confirmed with npm pack --dry-run.

Verification

	typescript 6.0.3	typescript 7.0.2
npm run build	exit 0	exit 0
built CLI runs	✅	✅

Packed tarball contains exactly cli.js, index.js, AgentManager-*.js, both .d.ts, and schema.graphql — no maps, 59.8 kB. Lint, format, and 53 files / 345 tests pass. Bundle is slightly smaller than tsup's (94 kB vs 105 kB).

_{🤖 Generated with Claude Code}

[gemini-code-assist]

Code Review

This pull request replaces the build tool tsup with tsdown, updating the build and dev scripts in package.json and adding a new tsdown.config.ts configuration file. It also updates the package exports to explicitly define type definitions. A critical issue was identified in the tsdown.config.ts file where the configuration option outExtensions is misspelled as plural instead of singular (outExtension). This typo would cause the option to be ignored, resulting in .mjs output files that break the package exports pointing to .js files.

[github-actions]

🎉 This PR is included in version 0.16.54 🎉

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