fix(examples): make MemoryCompactionExample show memory files and fire compaction

[147cai]

What

MemoryCompactionExample is meant to demonstrate two things (per its Javadoc):
conversation compaction, and the two-layer memory files it produces. In practice
neither was observable when running the example:

1. The "Check generated memory files" section always printed
   (memory/ directory not yet created) and never showed any file content.
2. Compaction never fired — the printed context size kept growing across turns and
   ★ compaction fired! was never reached — despite the Javadoc stating
   triggerMessages=6, keepMessages=2 should fire compaction after a few turns.

Root cause

(1) Memory files are written through WorkspaceManager, which applies a
runtime-data namespace from the RuntimeContext. The default IsolationScope.USER
falls back to the sessionId when no userId is set, so files land under
<workspace>/<sessionId>/memory/ and <workspace>/<sessionId>/MEMORY.md. The
example read them from the workspace root, so the lookup never matched. The check
also ran before the asynchronous flush was awaited.

(2) Compaction uses keepTokens, which defaults to dynamic mode (-1).
CompactionMiddleware resolves it from the model context window (qwen-plus =
131072) to min(8000, max(2000, usable * 0.25)) = 8000 tokens. Because
keepTokens > 0, the cutoff logic uses the token-based keep window and ignores
keepMessages. This short demo conversation is only ~1,000 tokens — far below the
8,000-token keep window — so the cutoff resolves to 0 and compaction is skipped
every time.

Fix

• Resolve memory-file paths through the namespace-aware WorkspaceManager
  (getMemoryDir(ctx) / resolveRuntimeDataPath(ctx, "MEMORY.md")), and move the
  existing Thread.sleep(3000) before the file check.
• Add .keepTokens(0) to force message-count keep mode, so compaction triggers
  after a few turns as the Javadoc describes.

Verification

Ran the example with qwen-plus before and after.

Memory files — after:
── Memory files on disk ──
2026-07-01.md
── MEMORY.md content ──

• Paris is the capital city of France.
• Tokyo, Japan has a population of approximately 14 million people.

Compaction — after:
Compaction triggered: total=25 msgs / 1087 tokens, cutoff=23, keeping=2 msgs
Compaction complete: 25 msgs → 1 summary + 2 tail = 3 total
context: 24 → 4 messages ★ compaction fired!

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