Move zk cards to docs/zk/ for documentation restructuring (#84)

## Summary
- Move all existing zettelkasten cards from `docs/` to `docs/zk/` as a temporary holding area
- Update `zk-docs` mise task to look in the new location
- Add `docs/README.md` explaining the Diataxis-based restructuring plan and target audiences

## Context
This is phase 1 of a multi-phase documentation restructuring effort. The goal is to reorganize docs to follow the Diataxis framework while serving multiple audiences:
1. Erich (owner) - knowledge graph/zk
2. Claude/AI agents - memory and context enrichment
3. New external readers - high-level overview
4. Potential operators/contributors - onboarding
5. Replicators - people wanting to duplicate the approach

## Testing
- [x] Verified `mise run zk-docs` still works with the new path
- [x] Updated obsidian.nvim config (in ~/.config/nvim) to point to new path

## Note
The obsidian.nvim config change is outside this repo but was made as part of this work.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Reviewed-on: https://forge.ops.eblu.me/eblume/blumeops/pulls/84

mise-tasks/zk-docs

@@ -4,7 +4,7 @@
 set -euo pipefail
 
 # Blumeops docs now live in the repo itself (symlinked into zk)
-DOCS_DIR="$(cd "$(dirname "$0")/.." && pwd)/docs"
+DOCS_DIR="$(cd "$(dirname "$0")/.." && pwd)/docs/zk"
 MAIN_CARD="$DOCS_DIR/1767747119-YCPO.md"
 
 # Find all files tagged with blumeops (excluding main card)