fix(memory): skip meta json files during session migration

[wj-xiao]

📝 Description

Skip *.meta.json files during session migration so startup does not treat JSONL metadata as legacy session snapshots. This prevents MigrateFromJSON() from overwriting existing .jsonl history with an empty message list and renaming metadata files to .meta.json.migrated.

🗣️ Type of Change

• 🐞 Bug fix (non-breaking change which fixes an issue)
• ✨ New feature (non-breaking change which adds functionality)
• 📖 Documentation update
• ⚡ Code refactoring (no functional changes, no api changes)

🤖 AI Code Generation

• 🤖 Fully AI-generated (100% AI, 0% Human)
• 🛠️ Mostly AI-generated (AI draft, Human verified/modified)
• 👨‍💻 Mostly Human-written (Human lead, AI assisted or none)

🔗 Related Issue

📚 Technical Context (Skip for Docs)

• Reference URL:
  • pkg/memory/migration.go
• Reasoning:
  • Startup migration scanned all *.json files, including *.meta.json.
  • meta.json belongs to the JSONL storage format, not the legacy session snapshot format.
  • Re-importing it caused SetHistory(..., emptyMessages) and cleared the paired .jsonl file.

🧪 Test Environment

• Hardware: PC
• OS: macOS
• Model/Provider: N/A
• Channels: Pico

📸 Evidence (Optional)

Click to view Logs/Screenshots

env GOCACHE=/tmp/picoclaw-go-build-cache GOTMPDIR=/tmp GOLANGCI_LINT_CACHE=/tmp/picoclaw-golangci-cache make lint
env GOCACHE=/tmp/picoclaw-go-build-cache GOTMPDIR=/tmp go test ./pkg/memory -run 'TestMigrateFromJSON_(Basic|WithToolCalls|MultipleFiles|InvalidJSON|RenamesFiles|Idempotent|ColonInKey|RetryAfterCrash|NonexistentDir|SkipsMetaJSONFiles)'

☑️ Checklist

• My code/docs follow the style of this project.
• I have performed a self-review of my own changes.
• I have updated the documentation accordingly.

[lxowalle]

LGTM