feat(packaging): add langfuse optional extra

[tt-a1i]

Fixes #48576

Summary

• Add a langfuse optional dependency extra for the bundled observability/langfuse plugin.
• Regenerate uv.lock, adding only langfuse==4.9.0 and its backoff==2.2.1 transitive dependency.
• Document the NixOS extraDependencyGroups = [ "langfuse" ] path for sealed uv2nix/container deployments.

Notes

• hermes-agent[langfuse] is intentionally not included in [all], so default installs remain unchanged.
• No LAZY_DEPS entry is added because the target use case is build-time inclusion in a sealed Nix venv, where runtime installation is not available.

Tests

• /tmp/hermes-uv-lock-venv/bin/uv lock --check
• python3.11 -c "...tomllib structure assertions..."
• UV_PROJECT_ENVIRONMENT=/tmp/hermes-langfuse-extra-import-env-rebase /tmp/hermes-uv-lock-venv/bin/uv run --extra langfuse --locked --no-dev python -c "from langfuse import Langfuse, propagate_attributes; print(Langfuse.__name__, callable(propagate_attributes))"
• git diff --check origin/main...HEAD

Not run

• cd website && npm run typecheck: local website dependencies are not installed (node_modules missing; Docusaurus packages unresolved).
• cd website && npm run build: local website dependencies are not installed (docusaurus: command not found); prebuild also reports missing Python yaml in this local environment.