chore(overlay): scaffold custom/ overlay tree and add microsoft_shared? predicate

Foundation for LeadChat customizations using Chatwoot's built-in custom/
extension mechanism (ChatwootApp.extensions). Adds:

- LEADCHAT.md: index of every customization touchpoint, updated as work lands
- custom/: overlay tree where Custom::Leadchat::* modules live, autoloaded
  alongside enterprise/ via a parallel block in config/application.rb
- Channel::Email#microsoft_shared? predicate as the first overlay (used by
  the upcoming Microsoft 365 Shared Inbox provider)

OSS-file additions wrapped in '# === LeadChat: ===' markers so future
upstream-merge conflicts are mechanical.

LEADCHAT.md

@@ -0,0 +1,49 @@
+# LeadChat customizations
+
+This file is the index of every LeadChat-specific customization on top of upstream Chatwoot. Consult it during upstream merges to know what's customized and where conflicts may surface.
+
+LeadChat is a white-label Chatwoot fork maintained on the `leadchat` branch. Customizations are layered using Chatwoot's built-in `custom/` extension mechanism (see [`lib/chatwoot_app.rb`](lib/chatwoot_app.rb) — `ChatwootApp.extensions` includes `'custom'` automatically when a `custom/` directory exists at repo root).
+
+## Layering strategy
+
+1. **Overlay modules under `custom/`** — preferred for Ruby modifications. New code lives in `custom/app/...` under the `Custom::Leadchat::*` namespace; OSS files only get a one-line `include_mod_with` / `prepend_mod_with` hook that Chatwoot's extension loader resolves automatically.
+2. **Direct edits to OSS** — only where no overlay mechanism exists (routes, schedule, Vue components). Each block is wrapped in `# === LeadChat: <feature> (start/end) ===` markers so future merge conflicts are mechanical.
+3. **Brand script** — separate process that overwrites brand strings in upstream files even after they're updated. Listed in "Brand script targets" below so we can extend its rules as new strings surface.
+
+## Customization categories
+
+### A. Overlay modules (Custom::Leadchat::*)
+
+| Hook in OSS file | Overlay file | Adds |
+|---|---|---|
+| `app/models/channel/email.rb` (bottom) | `custom/app/models/custom/leadchat/channel/email_extension.rb` | `Channel::Email#microsoft_shared?` predicate |
+
+### B. Direct edits to OSS files
+
+| File | Change | Marker |
+|---|---|---|
+| `config/application.rb` | Mirror enterprise/ autoload setup for `custom/` so `Custom::*` modules are autoloaded and `custom/config/initializers/**/*.rb` are required | `# === LeadChat: custom/ overlay autoload ===` |
+| `app/models/channel/email.rb` (bottom) | One-line `include_mod_with` hook for the email_extension overlay | `# === LeadChat: microsoft_shared ===` |
+
+### C. New top-level files (no upstream conflict potential)
+
+| Path | Purpose |
+|---|---|
+| `LEADCHAT.md` | This file |
+| `custom/` | Root of LeadChat overlay tree, picked up by `ChatwootApp.custom?` and the Phase-0 autoload setup |
+
+### D. Brand script targets
+
+Strings/files that the brand script rewrites away from "Chatwoot". Extend this list whenever a new "Chatwoot" mention surfaces in product UI.
+
+- `app/javascript/dashboard/routes/dashboard/settings/inbox/components/SenderNameExamplePreview.vue` lines 33, 43 — hardcoded `businessName: 'Chatwoot'` in sender-name preview tiles. (Discovered 2026-04-27 while building the Microsoft Shared Inbox feature; not yet handled by the brand script.)
+
+## Active feature work
+
+- **Microsoft Shared Inbox provider** (in progress) — new `microsoft_shared` Email Channel type backed entirely by Microsoft Graph API, supports multiple shared mailboxes per OAuth via separate channel-add flows, bypasses Security Defaults / SMTP AUTH retirement. Plan: `~/.claude/plans/starry-mapping-snowflake.md`.
+
+## Conventions
+
+- Conventional Commits (`type(scope): subject`)
+- No Claude attribution in commits or PR bodies
+- Update this file as each customization lands, not as a final pass