chore: release oxana 2.1.3

[reneklacan]

Summary

• bump all Oxana workspace crates to 2.1.3
• update workspace dependency constraints, lockfile, and README install command
• promote the worker error formatting changelog entry to the 2.1.3 release

Verification

• cargo check --all
• git diff --check

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Note

Low Risk
Mechanical version and documentation updates with no runtime or API code changes in this diff.

Overview
Release 2.1.3 — version-only change across the workspace (oxana, oxana-macros, oxana-web), root Cargo.toml dependency pins, Cargo.lock, and the README cargo add example.

The 2.1.3 changelog section documents worker error storage via default Debug formatting (backtraces on retry, dead, and drained jobs) and RuntimeBuilder::error_formatter; the wording now explicitly includes drained jobs versus the prior unreleased note.

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