docs(installer,worker): document Environment Checks + note unmerged prerequisite gap

Documents the Checks/ + SystemCheckPage feature (gating rule, check list,
FreshInstall-only placement) and the ExecutableResolver/.cmd-shim wiring in
ClaudeDo.Worker's ClaudeCliPreflight/ClaudeProcess. Adds docs/explore-notes/installer-preflight.md
(bumped/corrected against the actual implementation) and links it from the
explore-notes README and root CLAUDE.md.

The underlying code lives only on unmerged task branches (06aca9b3.../40272c0b...),
not on main yet, and two follow-up features ("Claude Help Me" button, Config-mode
Diagnose section) were never implemented because they blocked on that same missing
merge. Both gaps are called out explicitly in the new docs and in docs/open.md's
verification checklist, instead of being asserted as done.

docs/explore-notes/README.md

@@ -19,6 +19,7 @@ These sit **between** the CLAUDE.md files and the code:
 | [external-mcp](external-mcp.md) | The `claudedo` MCP tool surface + its two test-enforced conventions |
 | [review-merge](review-merge.md) | Approve=merge-unit, verify gate, `MergeCommit`/revert, diff stack, conflict resolver |
 | [conpty-sessions](conpty-sessions.md) | Interactive/planning/list-handler launch specs + the arg-flattening gotcha |
+| [installer-preflight](installer-preflight.md) | CLI version/login/auto-mode research, the `ExecutableResolver`/shim root cause, and the Installer's `Checks/`+`SystemCheckPage` implementation status |
 
 ## Rules