docs: update install commands; drop skip-locked keyword

[NikolayS]

Summary

The three client libraries (pgque-py, pgque, pgque-go) are now published at v0.2.0-rc.1 to PyPI, npm, and the public Go module mirror. The READMEs still said "after the first ... release" — fixed.

Changes

• `README.md` (top-level): drop "after the first ... release" comments from each install snippet; replace with the actual rc-aware install commands.
• `clients/python/README.md`: `pip install --pre pgque-py` (or pin `==0.2.0rc1`). `--pre` is needed because pip skips prereleases by default.
• `clients/typescript/README.md`: `npm install pgque@rc`. The `@rc` dist-tag will keep working through rc.N drops; users can drop the tag once v0.2.0 ships.
• `clients/go/README.md`: `go get github.com/NikolayS/pgque-go@v0.2.0-rc.1`. `@latest` also works while the v0.2.0 line is in rc.
• `clients/typescript/package.json`: drop `skip-locked` from keywords. Misleading — pgque uses PgQ's tick-based snapshot visibility, not FOR UPDATE SKIP LOCKED (that's pgmq's mechanism). Replaced with `background-jobs` (already a keyword in pgque-py's metadata for cross-ecosystem consistency).

Validation

End-to-end smoke tests against published packages all pass:

Registry	Install command tested	Send/Receive/Ack	Stale ack rowcount
PyPI	`pip install --pre pgque-py==0.2.0rc1`	✅	✅ 0
npm	`npm install pgque@rc` (then ESM `import`)	✅	✅ 0
Go module proxy	`go get github.com/NikolayS/pgque-go@v0.2.0-rc.1`	✅	✅ 0

The keyword removal only takes effect on the next npm publish (rc.2 / GA); rc.1 retains `skip-locked` on its registry page until then.

Test plan

• CI green
• Verify rendered READMEs on the PR's diff view look correct

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