[codeforstartups]

Closes #290. Supersedes #336 by @shivamm-gupta (credited as co-author). #336 was written against the agent loop before #328/#345, so it conflicts in react.py and drops the shared-state context. This ports it onto the current loop.

Stacked on #348 (HITL interrupts) — both change react.py. Review/merge #348 first; GitHub will retarget this PR to development automatically when #348's branch is deleted.

What it does

from dynavec.agents import ReActAgent, DynamoDBMemoryStore, WindowMemoryPolicy

memory = DynamoDBMemoryStore(table, namespace="app", policy=WindowMemoryPolicy(k=20))
agent = ReActAgent(model, memory=memory, system_prompt="Helpful assistant.")
agent.run("My name is Alice.", thread_id="u-42")
agent.run("What is my name?", thread_id="u-42")   # sees the earlier turn

From #336, unchanged: InMemoryMemoryStore, DynamoDBMemoryStore, window / token-budget / summary policies, scratchpad, and its tests.

Fixes made while porting

• Scratchpad could be overwritten: DynamoDBMemoryStore.append read the item twice and rewrote it with put_item, so a set_scratchpad in between was lost. It now uses update_item on the message attributes only.
• 400 KB item limit: each thread is one item; documented, with a pointer to the trimming policies.
• Shared state kept: the user turn still carries the Issue 289 supervisor v2 #345 AgentState context.
• Memory + checkpointer on one thread_id: the second run() used to reload the finished checkpoint instead of starting a new turn. A finished checkpoint now starts a new turn; unfinished ones (e.g. max_steps_reached) still resume as before — the existing resumption test is unchanged.
• Works with interrupts: a paused turn records the user message once, and the final answer is recorded after resume().

Tests

#336's 7 tests kept, plus 6 new: scratchpad survives append, item metadata, shared-state context in the stored turn, memory + checkpointer multi-turn, finished checkpoint starts fresh, memory with interrupt/resume. Local: ruff clean, strict mypy clean, 1137 passed.

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