feat(hint): H2 Phase C batch 1 — app-command tier-3 hints (ADR-0053)

Per-form hints for the 14 app-lifecycle commands (quit/help/hint/
rebuild/save/new/load/export/import/mode/messages/undo/redo/copy),
reference-leaning what/example with concept where it teaches (rebuild,
mode, messages, undo, export, help). hint_ids wired, catalogue + keys.rs
registered. +1 spot test; 2489 pass / 1 ignored, clippy clean.

src/friendly/strings/en-US.yaml

@@ -406,6 +406,56 @@ hint:
       what: "Link two tables so a parent row can own many child rows."
       example: "add 1:n relationship from Customers.id to Orders.customer_id"
       concept: "The \"1:n\" means one parent, many children. The child column holds the foreign key; add `--create-fk` to create that column if it doesn't exist yet."
+    # App-lifecycle commands (Phase C batch 1). Reference-leaning, so
+    # `concept` appears only where there's a real idea to teach.
+    quit:
+      what: "Leave the playground. Your project is already saved to disk."
+      example: "quit"
+    help:
+      what: "List every command, or show the detail for one."
+      example: "help insert"
+      concept: "`help` is the reference; press F1 while typing for a hint about the command you're building right now."
+    hint:
+      what: "Explain the most recent error — or, pressing F1 while typing, the command you're building."
+      example: "hint"
+    rebuild:
+      what: "Rebuild the project database from its saved text files."
+      example: "rebuild"
+      concept: "The text files (project.yaml + the data folder) are the source of truth; the database is derived and can always be rebuilt from them."
+    save:
+      what: "Save the current project under a name; `save as` copies it to a new one."
+      example: "save as my-shop"
+    new:
+      what: "Close the current project and start a fresh temporary one."
+      example: "new"
+    load:
+      what: "Open the project picker to switch to a saved project."
+      example: "load"
+    export:
+      what: "Write a shareable zip of the project — its text files only, never the database."
+      example: "export my-shop.zip"
+      concept: "The zip carries the schema and data as text, so anyone can rebuild the very same database from it."
+    import:
+      what: "Unpack a project zip into a new project and switch to it."
+      example: "import my-shop.zip as shop-copy"
+    mode:
+      what: "Switch between simple mode (the guided teaching commands) and advanced mode (raw SQL)."
+      example: "mode advanced"
+      concept: "Simple mode uses keyword commands; advanced mode lets you write SQL directly. A leading `:` runs a single advanced command without switching modes."
+    messages:
+      what: "Show or set how much detail error messages give."
+      example: "messages short"
+      concept: "Verbose (the default) adds a fix-it hint under each error headline; short shows just the headline."
+    undo:
+      what: "Undo the most recent change, after a confirmation."
+      example: "undo"
+      concept: "Every data or schema change is snapshotted first, so you can step back; `redo` re-applies what you undid."
+    redo:
+      what: "Re-apply the most recently undone change."
+      example: "redo"
+    copy:
+      what: "Copy the output panel to the clipboard — all of it, or just the last command's output."
+      example: "copy last"
   err:
     foreign_key:
       child_side: