feat: Implement Slack export importer and enhance README documentation for historical context recovery

archives/slack/README.md

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+# Slack Archive
+
+Use this directory for historical Slack exports that should be available to the workspace when old evidence is needed.
+
+## Recommended Layout
+
+Preferred:
+
+- `archives/slack/export/`
+  Extracted Slack export directory
+
+Optional:
+
+- `archives/slack/export.zip`
+  Raw zip kept for reference only
+
+## Important
+
+- The current importer works with the extracted export directory, not the zip directly.
+- Keep raw archive content here, not under `ai/` or `knowledge/`.
+- This directory is ignored by git except for this README and `.gitignore`.
+
+## Why Here
+
+This keeps:
+
+- raw historical evidence separate from curated memory
+- project memory clean
+- historical context available when the agent needs to recover old Jira, people, or decision context