wiki: document homelab-context distribution system

Adds infrastructure/homelab-context.md as the architecture reference for
the cross-client context + MCP + secrets-issuance system. Updates:

- 105-apps.md: two new ## Stacks sections (homelab-mcp, secrets-issuance)
  with their deploy pipelines + a row each in the public-hostname table;
  changelog entry.
- auto-deploy.md: both new pipelines added to the table (one repo, two
  webhooks, same push); per-pipeline notes covering the clone-per-service
  pattern and the deploy.sh self-restart caveat; changelog entry.
- README.md: link to the new infrastructure page.

Operational walkthrough already lives at operations/agent-enrollment.md;
this commit is the architecture side of the same story.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

README.md

@@ -43,6 +43,7 @@ See the full table in [`containers/index.md`](containers/index.md). Quick links:
 - [Backups — restic on external drive (disabled)](infrastructure/backups.md)
 - [Auto-deploy — gitea-webhook pipelines](infrastructure/auto-deploy.md)
 - [VPS hardening — IONOS / netbird control plane](infrastructure/vps-hardening.md)
+- [Homelab context distribution](infrastructure/homelab-context.md) — cross-client `/opt/homelab-context` + MCP + secrets-issuance
 
 ### Investigations
 Time-stamped incident notes / experiments in [`investigations/`](investigations/index.md).