Fetch only on request, remove --no-fetch

Nothing is fetched on startup any more; the header and detail pane show
the last fetch and f fetches everything. Records the change as D22.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

docs/README.md

@@ -31,8 +31,7 @@ Installs `submarine` and the alias `sub` (linux/macOS, amd64/arm64) to
 
 **Prototype.** The UI is complete for browsing. It reads real repositories
 through a git provider (`internal/submodule/git`): `submarine [dir]`. It
-fetches in the background on startup (`--no-fetch` to skip; `f` to fetch
-again) and follows changes made in other shells. `submarine --mock` shows
+fetches only when asked (`f`) and follows changes made in other shells. `submarine --mock` shows
 the built-in mock data (`internal/submodule/mock`). `!` shows only
 submodules with problems. The actions `u` and `b` only show a "not
 implemented" message ([drafts of the write actions](actions.md)).

docs/architecture.md

@@ -119,9 +119,9 @@ The mock provider (`internal/submodule/mock`) covers every state and sleeps
    calling `Status()` → `statusLoadedMsg`.
 3. Each result is stored on the submodule (`Status` or `Err`); the view
    updates incrementally, including parents' hidden-issue counts.
-4. After the first tree: the watcher starts (`changeMsg`), and the
-   background fetch starts one `tea.Cmd` per fetchable repository
-   (`fetchDoneMsg`).
+4. After the first tree: the watcher starts (`changeMsg`). Nothing is
+   fetched until the user presses `f`, which starts one `tea.Cmd` per
+   fetchable repository (`fetchDoneMsg`).
 5. The spinner ticks only while something loads or fetches.
 
 ### Fetch and refresh are decoupled (D20)
@@ -197,7 +197,7 @@ No CI runner: releases are built and published from a developer machine.
   so `sub` works too). `SUBMARINE_UPDATE_URL` overrides the registry.
 - After the TUI exits, a one-line notice is printed if a newer release
   exists. The check runs in the background at startup, at most once a day
-  (cached in the user cache dir) and is off for dev builds, `--no-fetch`,
+  (cached in the user cache dir) and is off for dev builds,
   `--mock` and `SUBMARINE_NO_UPDATE_CHECK=1`.
 
 ## Testing

docs/decisions.md

@@ -150,7 +150,7 @@ removed + untracked.
 **Consequences.** One extra `git diff` per dirty submodule, none for clean
 ones. Not included: staged/unstaged markers, diff bodies, stash count.
 
-## D18 — Background fetch on startup
+## D18 — Background fetch on startup (superseded by D22)
 
 **Context.** Without fetching, every answer is only as fresh as the user's
 last `git fetch`, which they rarely run in every submodule.
@@ -296,3 +296,20 @@ count as problems because their state is unknown (D7 counts `error` as danger
 too); the cost is that rows vanish as statuses arrive, so the selection is
 refitted after each status. Staged pins are not shown by it, since they are
 not issues.
+
+## D22 — Fetch only on request
+
+**Context.** D18 fetched every repository on startup. With hundreds of
+submodules that is hundreds of network round trips (and credential or
+host-key failures) every time the tool opens, whether or not the user wants
+fresh remote data.
+**Decision.** Supersedes D18. Nothing is fetched on startup. The header and
+the detail pane show the last fetch (`FETCH_HEAD`, also from fetches made in
+other shells), so the user can judge how stale the remote refs are and press
+`f` to fetch. The `--no-fetch` flag is removed with it (it still had the
+side effect of switching off the update check; that has
+`SUBMARINE_NO_UPDATE_CHECK`).
+**Consequences.** Right after startup, `unpushed`, `↓n` and the like are as
+fresh as the last fetch. `fetched never` appears for a repository that was
+never fetched. The progress display and per-submodule fetch errors stay as
+they are, they now belong to `f`.

docs/requirements.md

@@ -100,9 +100,10 @@ commit hashes. It doesn't answer the questions users actually have:
 
 ### Fetch and refresh
 
-- **F50** On startup, after the local state is shown, the superproject and
-  every initialized submodule are fetched in the background (all remotes,
-  with prune). `--no-fetch` turns this off.
+- **F50** Nothing is fetched on startup. The header and the detail pane show
+  when the repositories were last fetched (`FETCH_HEAD`), whoever did it.
+  `f` fetches the superproject and every initialized submodule in the
+  background (all remotes, with prune).
 - **F51** While fetching, the header shows progress (`fetching 12/174`)
   instead of the fetch age; afterwards the age and the number of failed
   fetches. The detail pane shows each submodule's last fetch and, if it
@@ -141,13 +142,12 @@ See [actions.md](actions.md) for the draft of the write actions.
 ```sh
 submarine [dir]     # the repository containing dir (default: current directory)
 submarine --mock    # built-in mock data
-submarine --no-fetch [dir]   # no background fetch on startup
 ```
 
 ## Non-functional requirements
 
 - **N1** Startup must not block on the network. The tool shows the local
-  state at once and fetches in the background (F50).
+  state at once and uses the network only when the user asks (F50).
 - **N2** Loading many submodules must not freeze the UI (async, per
   submodule).
 - **N3** Colors follow the user's terminal theme (ANSI 16 colors); the UI