refactor: remove unused exports and dead code

[kojiwakayama]

Summary

• Removed export keyword from 350 declarations across 180 files that have zero external consumers (not imported by any other file in src/ or tests/)
• Deleted 2,080 lines of completely dead code (47 functions/classes/constants that were neither exported nor used internally), plus cascading dead infrastructure
• Zero new lint errors, zero new type errors, no test regressions

Approach

1. Extracted all ~4,800 non-barrel exports across src/
2. For each export, checked with rg -w whether any other file references it
3. Removed export keyword from 350 unused exports
4. Linter then revealed 47 of those were entirely dead code — deleted them
5. Cleaned up cascading dead code (e.g., testing/isolation.ts lost ~600 lines of env overlay infrastructure only reachable through the removed installTestIsolation)

Modules affected

Module	Exports removed	Dead code deleted
transforms	57	~400 lines
studio	51	—
server	37	~50 lines
utils	24	~220 lines
rendering	24	—
build	22	~215 lines
platform	21	~10 lines
agent	18	~60 lines
workflow	15	~195 lines
testing	2	~600 lines
other (12 modules)	79	~330 lines

Test plan

• deno lint src/ — zero errors
• deno check src/index.ts — no new type errors (3 pre-existing generated file errors unchanged)
• deno test src/ — no test regressions (299 pass vs 298 on main)