ci: add test + coverage workflow on main and PR

[elecnix]

What

Adds a GitHub Actions workflow (.github/workflows/test.yml) that runs on every push to main and every PR targeting main.

How

• Node.js 22 built-in test runner: node --experimental-strip-types --test
• Coverage via --experimental-test-coverage flag (also built-in, no external deps)
• Uses actions/checkout@v4 and actions/setup-node@v4

Also removes the redundant parse/syntax smoke test from test/agent-session.test.ts — the CI workflow now catches syntax errors without the subprocess-spawn hack.

Coverage output (example)

# file                   | line % | branch % | funcs % | uncovered lines
# agent-identity         |        |          |         |
#  daemon-client.ts      |  93.33 |    66.67 |   75.00 | 31-32 65-67
# test                   |        |          |         |
#  agent-session.test.ts |  98.13 |    73.91 |  100.00 | 73-74
# all files              |  96.15 |    70.73 |   90.91 |