chore: intent.txt を追跡対象から外して gitignore に追加

[yuyakinjo]

変更内容

• intent.txt（Intent-First Development の原典ドラフト）をリポジトリの追跡対象から削除し、.gitignore に追加
• intent.txt の実在を前提としていた test/docs/references.test.ts のテスト（ADR 48 関連）を削除

背景

intent.txt は手元にだけ置いておくためのファイルとし、リポジトリには含めないようにする。

[github-actions]

Startup benchmark

hello world, 50 warm runs. Both builds are measured in the same
job on the same runner, so the machine cancels out of the comparison.

Build	Mean	Own share
this PR	19.9 ms	16.7 ms
main	19.7 ms	16.5 ms
empty Bun binary (runtime floor)	3.2 ms	—

Difference: +0.2 ms (+1.1%) against main.

Own share is the mean minus the runtime floor — the part this project is
responsible for. CI runners are noisy; read anything under a few percent
as noise rather than a regression.

Bundle size

Minimal CLI (test/fixtures/hello-app: one command, <Line> only),
decopin build --minify. Framework code is all of it, so growth here is
framework growth.

Build	Size
this PR	82.2 KB
main	82.2 KB

Difference: +0.0 KB (+0.0%) against main.
Bytes are deterministic: any difference is real, not runner noise.