docs: README + SPDX license / build-warning cleanup

[nyorf]

Packaging polish from the first real release build (1.1.1):

• README.md -- overview, tag-pin install, quickstart (HTTP service, outgoing-call propagation, scripts/workers), the setup_logging env-var table, and dev commands. Also clears the sdist 'standard file not found: README' warning.
• SPDX license -- `license = { text = "MPL-2.0" }` -> `license = "MPL-2.0"`, with `setuptools>=77` so the build env understands it. Removes the `project.license` TOML-table deprecation warning (deprecated by setuptools, removed by 2027-Feb-18).

Verified in a clean-room build: no deprecation/README warnings, README bundled in the sdist, ruff clean, 37 passed / 1 xfailed.