Workspace isolation

# Agent A — workspace "acme"
# PLYRA_API_KEY=plm_live_acme...
async with Memory(agent_id="agent-1") as mem:
    await mem.remember("acme internal strategy")

# Agent B — workspace "other"
# PLYRA_API_KEY=plm_live_other...
async with Memory(agent_id="agent-1") as mem:
    ctx = await mem.context_for("acme internal strategy")
    # → returns nothing. Workspace isolation enforced server-side.

# Support agent — only sees its own memory
async with Memory(agent_id="support-agent") as mem:
    await mem.remember("user prefers dark mode")

# Sales agent — different namespace, cannot see support memory
async with Memory(agent_id="sales-agent") as mem:
    ctx = await mem.context_for("user preferences")
    # Returns nothing — different agent_id namespace