refactor(pocket-pi): TypeScript build pipeline for the JS guest layer

[doodlewind]

Phase 3 of the effort (follows the pi 0.81 bump + self-contained binary in #3). The JS glue was hand-dropped .js; now it's TypeScript under js/src/, compiled by one build step.

Does this affect future pi bumps? No.

The glue is Pocket Pi's own runtime layer, separate from pi (still a real, unmodified npm dependency that gets bundled). A bump stays npm update + rebuild + (if the newer pi needs a new builtin/global) add a shim — and the shim being .ts instead of .js changes nothing. The bundle output is JS regardless (esbuild takes a .ts entry). If anything, the typecheck makes the "fix what breaks" step safer.

Layout

js/src/runtime/**  → crates/pocket-pi/js/**             (per-file transpile; include_str!'d)
js/src/trimmed/**  → crates/pocket-pi/js/agent.bundle.js
js/src/pi-full/**  → crates/pocket-pi/js/pi-full.bundle.js(.gz) + harness scripts

• One node js/build.mjs runs tsc --noEmit (typecheck) + esbuild (transpile the runtime glue, emit the harness scripts, bundle the trimmed + full pi). esbuild and typescript are real devDeps now (no more npx esbuild@…); build-pi-full.mjs folded in.
• Rust include_str! paths are unchanged — the build emits .js exactly where they're expected. The emitted runtime .js + agent.bundle.js are committed so cargo still builds Rust-only; the full-pi bundle stays git-ignored.
• tsconfig typechecks Pocket Pi's own orchestration code (the pi-full session harness). The runtime polyfills and the pi-ai/undici adapter glue are transpile-only — typechecking a polyfill against the very APIs it reimplements is counterproductive. env.d.ts declares the loose host/global surface.
• The trimmed bundle now builds against pi 0.81 too (the pi-ai stub gains contentText/retryAssistantCall/uuidv7, which 0.81's pi-agent-core imports).

Tests / CI

Full suite green — 23 unit tests + 8 #[ignore] (bundle load, extension bind, extension tool turn, persistence, embedded full pi); tsc + clippy -D warnings clean.

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