docs: add hook lifecycle stages and BaseTransactionTool guidance to PLUGINS.md

#353 (predicates epic) must be fully complete before starting this task. T3 (#357) must also be complete — this doc references `BaseTransactionTool` and `BaseQueryTool` introduced there.

JS reference: [PR #720](https://github.com/hashgraph/hedera-agent-kit-js/pull/720)

## Scope

- Add 4-stage hook lifecycle section to `docs/PLUGINS.md`:
1. `pre_tool_execution`
2. `post_params_normalization`
3. `post_core_action`
4. `post_secondary_action`
- Clarify which stages fire for `BaseTransactionTool` vs query tools; note that plain `Tool` subclasses do not participate in hooks
- Add `examples/plugin/smoke_test.py` mirroring the JS lifecycle test pattern
- Cross-link from main hooks docs

First verify that #353 and #357 are complete, then read docs/PLUGINS.md and the existing hook documentation. Use the JS lifecycle test pattern referenced in PR #720 to shape examples/plugin/smoke_test.py. Done means the four stages, tool participation rules, and cross-links are documented and the smoke test covers the lifecycle.

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