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.

src/ClaudeDo.Worker/CLAUDE.md

@@ -16,7 +16,8 @@ Worker/
   State/        — TaskStateService + TransitionResult (sole owner of Status/PlanningPhase/BlockedBy writes)
   Queue/        — IQueueWaker, IQueuePicker, QueueService, OverrideSlotService, RunCancellationRegistry
   Lifecycle/    — StaleTaskRecovery, TaskResetService, TaskMergeService, VerifyCommandRunner,
-                  ClaudeCliPreflight, OrphanRecovery, PlanningLineageRecovery,
+                  ClaudeCliPreflight (resolves via ExecutableResolver, see Key Components below),
+                  OrphanRecovery, PlanningLineageRecovery,
                   AttachmentOrphanRecovery, PromptFileRecovery  (last four = startup sweeps)
   Worktrees/    — WorktreeMaintenanceService
   Agents/       — AgentFileService, DefaultAgentSeeder
@@ -111,7 +112,8 @@ Full flow, invariants, and model/effort/max-turns resolution (including the low-
 
 ## Key Components
 
-- **ClaudeProcess** — spawns `claude -p --output-format stream-json --verbose --permission-mode auto` (or whatever app settings specify). Prompt via stdin, NDJSON from stdout. CancellationToken kills the process tree.
+- **ClaudeCliPreflight** — startup check that `claude --version` runs; resolves the binary via `ExecutableResolver` first and returns a clear "not found on PATH" result instead of a raw `Process.Start` exception when it doesn't. *(Not yet on `main` — see below.)*
+- **ClaudeProcess** — spawns `claude -p --output-format stream-json --verbose --permission-mode auto` (or whatever app settings specify). Prompt via stdin, NDJSON from stdout. CancellationToken kills the process tree. Resolves `_cfg.ClaudeBin` via `ExecutableResolver` (`ClaudeDo.Data`, shared with `ClaudeCliPreflight` and the Installer's checks) before spawning — a `.cmd`/`.bat` shim (e.g. an npm-installed `claude`) is launched through `cmd.exe /c` since `UseShellExecute = false` can't exec a shim directly; a resolved `.exe` starts exactly as before. Throws if nothing resolves. *(Not yet on `main` — see `Environment Checks` in `ClaudeDo.Installer/CLAUDE.md`.)*
 - **ClaudeArgsBuilder** — `--model`, `--effort`, `--max-turns`, `--append-system-prompt`, `--agents`, `--json-schema`, `--resume`
 - **StreamAnalyzer** — parses NDJSON; extracts session_id, token counts, turn counts, result text, structured output. Replaced MessageParser.
 - **WorktreeManager** — worktrees on `claudedo/{taskId[:8]}` branches; commits with semantic messages, updates DB with head commit + diff stats