fix: persist context key locally and remove dead node re-encryption

[LiranCohen]

Summary

• Persist context encryption key to network.json so non-anchor nodes survive DWN outages at startup. Previously, the context key was fetched from the anchor's DWN on every meshd up — if the DWN was unreachable, the node couldn't encrypt/decrypt records.
• Remove dead node re-encryption block in cmdUp — this code attempted to re-register the node with Protocol Context encryption on every startup, but it was dead code after PR fix: use Protocol Context encryption for all records so peers can decrypt (#87, #88) #89: anchor nodes already write with context encryption in network create, and non-anchor nodes' records are created by peer add (also with context encryption). Additionally, the DWN rejects updates from non-authors, so the non-anchor path always failed silently.
• Add cached-first pattern to acl show — same optimization: try cached context key before fetching from DWN.

Changes

File	Change
internal/state/state.go	Add ContextKey string field to NetworkState (base64-encoded X25519 private key)
cmd/meshd/main.go	cmdNetworkJoin: persist fetched context key to network.json
cmd/meshd/main.go	cmdUp: load context key from cache first, fall back to DWN fetch; persist newly fetched keys
cmd/meshd/main.go	cmdUp: remove 35-line dead node re-encryption block
cmd/meshd/main.go	cmdACLShow: cached-first context key loading (skip anchor, which derives via HKDF)

Verification

• go build ./... — zero errors
• go vet ./... — zero warnings
• go test ./... -count=1 -race — all tests pass, no data races