Add Pushary customer approval integration

[aadilghani1]

Adds Pushary as an externally maintained custom component, following the repository's draft integration page.

PusharyProtectedAction requests customer phone approval and consumes a one-use execution permit before invoking an application-owned action. The public MIT package is published on PyPI as pushary-haystack==0.1.0 and includes a Git tag, repository/issues links, a restartable local-order demo, and offline checks. It supports Haystack 3.1+ and can be used through Pipeline or ComponentTool.

Validation:

• Installed pushary-haystack==0.1.0 directly from PyPI into a fresh environment; dependency and real-framework checks passed.
• Built this page in the current haystack-home site with Hugo extended 0.155.2, matching preview-build.sh.
• Executed the documented Python example against Haystack 3.1.1 and Pushary 2.1.1 with simulated HTTP responses.
• Tested native pipeline serialization and fresh-process approval/refusal, changed inputs/identity, duplicate/concurrent execution, lost responses and uncertain effects.

Only integrations/pushary.md changes. The package is maintained in the Pushary organization; no dependency or runtime code is added here. Phone delivery needs a Partner account and enrolled customer; no live-phone test is claimed. This contribution was prepared with AI assistance and validated by the checks above.

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