add .gitignore, clean up generated files from node_modules

Remove from tracking:
- dist/ (build output, regenerated by bun run build.ts)
- node_modules/.pnpm/ (pnpm internal store, 247MB)
- node_modules/.bin/ (executable symlinks)
- node_modules/.ignored/ and .ignored_* (source-map dirs moved aside by pnpm)
- 100 pnpm-installed package symlinks (regenerated by pnpm install)

Keep in tracking:
- Private package stubs not on npm (color-diff-napi, modifiers-napi,
  @ant/claude-for-chrome-mcp, @anthropic-ai/mcpb, @anthropic-ai/sandbox-runtime)
- Source-map extracted packages not installed by pnpm (@grpc/*, @protobufjs/*, etc.)

claude-code-source/node_modules/@smithy/util-body-length-browser/dist-cjs/index.js

@@ -1,30 +0,0 @@
-'use strict';
-
-const TEXT_ENCODER = typeof TextEncoder == "function" ? new TextEncoder() : null;
-const calculateBodyLength = (body) => {
-    if (typeof body === "string") {
-        if (TEXT_ENCODER) {
-            return TEXT_ENCODER.encode(body).byteLength;
-        }
-        let len = body.length;
-        for (let i = len - 1; i >= 0; i--) {
-            const code = body.charCodeAt(i);
-            if (code > 0x7f && code <= 0x7ff)
-                len++;
-            else if (code > 0x7ff && code <= 0xffff)
-                len += 2;
-            if (code >= 0xdc00 && code <= 0xdfff)
-                i--;
-        }
-        return len;
-    }
-    else if (typeof body.byteLength === "number") {
-        return body.byteLength;
-    }
-    else if (typeof body.size === "number") {
-        return body.size;
-    }
-    throw new Error(`Body Length computation failed for ${body}`);
-};
-
-exports.calculateBodyLength = calculateBodyLength;