feat: add MaxPromptBlockChars to MemoryConfig and plumb it through MemoryStore

[fuseraft]

MemoryStore.BuildPromptBlock* hard-codes a 8,000-character cap on how much memory is injected into the system prompt. The value is never exposed to the user.

CompactionConfig.MaxCharsPerHistoryMessage is the direct precedent: same default, same rationale (token budget), already documented and user-facing. MemoryConfig (src/Core/Models/MemoryConfig.cs) exists but only covers Provider and Webhook — it has no tuning knobs.

Suggested change:

1. Add a property to MemoryConfig:

   /// <summary>
   /// Maximum characters injected into the system prompt from the memory store.
   /// Default: <c>8000</c> (~2 000 tokens). Raise for large-context models; lower for small ones.
   /// </summary>
   public int MaxPromptBlockChars { get; init; } = 8_000;

2. Update MemoryStore.BuildPromptBlock and BuildPromptBlockAsync to accept an optional int maxChars = 8_000 parameter.

3. Pass the config value at the two call sites:

   • src/Infrastructure/AgentFactory.cs:124 — MemoryStore.ForAgent(config.Name).BuildPromptBlock()
   • src/Cli/Commands/Repl/ReplCommand.cs:167 — memoryStore.BuildPromptBlockAsync(cwd)

   Both callers have access to a MemoryConfig or can reach it via OrchestrationConfig — check how CompactionConfig.MaxCharsPerHistoryMessage is accessed at its call sites for the right pattern.

4. Add a test case to tests/FuseraftCli.Tests/MemoryStoreTests.cs verifying that the block is truncated at the configured limit.

Files to touch:

• src/Core/Models/MemoryConfig.cs
• src/Infrastructure/MemoryStore.cs
• src/Infrastructure/AgentFactory.cs
• src/Cli/Commands/Repl/ReplCommand.cs
• tests/FuseraftCli.Tests/MemoryStoreTests.cs