use particular fromTag in a3p-integration (#10056)

_incidental_

## Description
Use a particular fromTag to avoid build errors when agoric-3-proposals advances `latest`.

Also changes the proposal id for `upgrade-next` from `e` to `n` to make it easier to remember, give ample namespace preceding it and make it very obvious from a proposal ID that it's after the next upgrade. E.g. `s:stake-bld` (which had been `g:stake-bld`)

### Security Considerations
n/a

### Scaling Considerations
n/a

### Documentation Considerations
Updated docs here. Agoric/agoric-3-proposals#180 should align.

### Testing Considerations
CI

### Upgrade Considerations
n/a

a3p-integration/README.md

@@ -46,7 +46,7 @@ cannot access SDK code. Their names must be lower case.
 In the release branches, the end-to-end `a3p-integration` test usually only has
 a single proposal package named `a:upgrade-NN`, which performs a chain software
 upgrade proposal to the corresponding upgrade plan. In the `master` branch, the
-next release's changes are staged in `a:upgrade-next`. There may also be
+next release's changes are staged in `n:upgrade-next`. There may also be
 core-eval proposal packages, either before or after the chain software upgrade
 proposal.
 
@@ -69,7 +69,7 @@ For a chain software upgrade proposal, the `type` is `"Software Upgrade Proposal
    `/golang/cosmos/app/upgrade.go`).
   - See **Generating core-eval submissions** below for details.
 
-For an (evolving) example, see `a:upgrade-next` in master.
+For an (evolving) example, see `n:upgrade-next` in master.
 
 ### Core-eval proposal
 

a3p-integration/package.json

@@ -1,7 +1,7 @@
 {
   "private": true,
   "agoricSyntheticChain": {
-    "fromTag": "latest"
+    "fromTag": "use-vaults-auctions"
   },
   "scripts": {
     "build": "yarn run build:sdk && yarn run build:submissions && yarn run build:synthetic-chain",

a3p-integration/proposals/README.md

@@ -18,7 +18,7 @@ Steps:
 We need the actual proposal that was sent to stakers. In the case of upgrades
 (chain-halting) it will be a dedicated release branch. (E.g.
 https://github.com/Agoric/agoric-sdk/tree/dev-upgrade-16/). If you're
-publishing an "upgrade" proposal and it's called `a:upgrade-next`, keep looking;
+publishing an "upgrade" proposal and it's called `n:upgrade-next`, keep looking;
 that is only a draft.
 
 ### Build the submissions
@@ -78,19 +78,25 @@ the repo. The PR's CI will test it and once the PR is merged it will update the
 
 ## How to revise this directory after 
 
-Once `latest` has changed, if it was a Software Upgrade Proposal then the
-upgrade handler in master will fail.  If that poses a problem, you can set
-a3p-integration `agoricSyntheticChain.fromTag` to a specific version instead
-of `latest`. See https://ghcr.io/agoric/agoric-3-proposals for the available
-tags.
+Once a new proposal is merged into agoric-3-proposals, take the proposal name
+(the part after the colon) and use it in the fromTag value in a3p-integration's
+package.json. For example, `a:my-proposal` becomes `"fromTag": "use-my-proposal"`.
 
-You need to revise `a:upgrade-next` to be able to apply on top of the last
-upgrade. In master it should already have these values, which should be
-maintained:
+The `agoricSyntheticChain.fromTag` should generally work with a value of 'latest',
+but that causes problems whenever agoric-3-proposals publishes a new image with changes
+that a3p-integration doesn't yet expect.
+
+So we specify a particular *use* image. E.g. `use-upgrade-16`.
+See https://ghcr.io/agoric/agoric-3-proposals for the available tags.
+
+If you're changing the fromTag to a a new SDK version (e.g. a new chain-halting
+upgrade) then you also need to revise the `upgrade-next` proposal to be able to
+apply on top of that upgrade. In master it should already have these values,
+which should be maintained:
 ```
     "releaseNotes": false,
     "sdkImageTag": "unreleased",
     "planName": "UNRELEASED_A3P_INTEGRATION",
 ```
 
-But you will have to remove from `app.go` whatever proposals were already executed.
+But you will have to remove from [upgrade.go](/golang/cosmos/app/upgrade.go) whatever proposals were already executed.