feat: add hierarchical context manager [Story 447.1]

Implements the hierarchical short/long-term SYNAPSE context manager for Epic 447, including concurrent mutation serialization, long-term summary preservation under hard token limits, registry/manifest updates, and regression coverage.

Author: rafaelscosta

.aiox-core/core/synapse/context/README.md

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+# SYNAPSE Context Runtime
+
+This directory contains bounded-context helpers for the SYNAPSE runtime.
+
+## HierarchicalContextManager
+
+`HierarchicalContextManager` keeps an LLM-ready message array under a configured
+token budget by moving older short-term messages into long-term summaries.
+Summarization is injectable and has a deterministic local fallback, so the
+manager can run in tests, CLI flows and offline agent loops without a live LLM.
+
+```javascript
+const { HierarchicalContextManager } = require('./.aiox-core/core/synapse/context');
+
+const contextManager = new HierarchicalContextManager({
+  maxTokens: 8192,
+  summarizationThreshold: 0.75,
+  tokenizer: text => Math.ceil(text.length / 4),
+  summarizer: async ({ messages }) => {
+    return messages.map(message => `${message.role}: ${message.content}`).join('\n');
+  },
+});
+
+contextManager.on('swap:complete', data => {
+  console.log(`Compacted ${data.messagesRemoved} message(s).`);
+});
+
+await contextManager.addMessage({ role: 'user', content: 'Build the next task.' });
+const safeContext = contextManager.getContext();
+```
+
+The manager does not call OpenAI, Anthropic, Claude, Gemini, Kimi or OpenRouter
+directly. Inject a summarizer when an agent loop wants model-based compression.
+If that summarizer fails, the manager emits `swap:error` and falls back to a
+local extractive summary instead of crashing the loop.