Add a complete runnable Postgres example (fixes #24)

[peczenyj]

Summary

Fixes #24.

This adds a complete, self-contained, runnable example under examples/pq, aimed at newcomers:

• app.go — ordinary application code (no copyist imports): a plain query function and a context.Context-aware variant.
• app_test.go — a heavily commented walkthrough of the whole workflow: Register, SetSessionInit, TestMain wiring, defer copyist.Open(t).Close(), and opening through the copyist_postgres driver. The comments are the documentation; the file reads top-to-bottom without needing any other file.
• testdata/app_test.copyist — a committed recording, so go test ./examples/pq passes immediately after cloning with no database, no Docker, no network. Re-recording is one command (go test ./examples/pq -record); the test starts a throwaway postgres:16 container itself via the existing drivertest/dockerdb helper, which also keeps make test (which deletes and re-records all recordings) working.
• A short README section pointing at the example.

It also answers the second question in #24: context.Context needs no mocking — copyist forwards contexts to the wrapped driver untouched, and TestQueryNameAt demonstrates a QueryRowContext call recorded and played back like any other.

Test Plan

• go test ./examples/pq passes with no database running (playback from the committed recording)
• go test ./examples/pq -record records against a real Postgres in Docker and regenerates the recording
• go test ./... at root passes (the example is part of the root module, so existing CI covers it on Go 1.18)
• gofmt/go vet clean

🤖 Generated with Claude Code

[cockroach-teamcity]

This change is 

[peczenyj]

Hey guys, anything wrong with this pull request?