Correct doc and help sentences that are false about the code (closes #233)

A blob is written in full to a temporary file in $TMPDIR and uploaded
once finished, not streamed to storage, and the metadata export works on
a copy of the local index there. The README, ARCHITECTURE.md and
config.example.yml now say a backup needs free space there of the larger
of blob_size_limit and about three times the size of the local index.
Also corrected: the snapshot ID format, what restore reads and how
incomplete snapshots are removed in docs/DATAMODEL.md, what source_path
holds, the index_path default, the config search order in the snapshot
create help, what snapshot remove cleans up in the prune help, how the
release installs Go, and the script/release and script/fmt-check
comments.

Model: opus-5-5

docs/DATAMODEL.md

@@ -111,7 +111,7 @@ Maps chunks to the blobs that contain them.
 Tracks backup snapshots.
 
 **Columns:**
-- `id` (TEXT PRIMARY KEY) - Snapshot ID (format: hostname-YYYYMMDD-HHMMSSZ)
+- `id` (TEXT PRIMARY KEY) - Snapshot ID (format: `hostname_name_timestamp`, e.g. `server1_home_2025-06-01T12:00:00Z`: the hostname up to its first `.`, the snapshot name, and an RFC 3339 UTC timestamp)
 - `hostname` (TEXT) - Hostname where backup was created
 - `vaultik_version` (TEXT) - Version of Vaultik used
 - `vaultik_git_revision` (TEXT) - Git revision of Vaultik used
@@ -218,8 +218,8 @@ The `{remote-key}` directory name is a one-way hash of the human snapshot ID, so
 ### 4. Restore Process
 
 The restore process doesn't use the local database. Instead:
-1. Downloads snapshot metadata from S3
-2. Downloads required blobs based on manifest
+1. Downloads and decrypts the snapshot's metadata database (`db.zst.age`) from S3
+2. Downloads the blobs holding the chunks of the files being restored, found through that database's `blob_chunks` table; the manifest is not read
 3. Reconstructs files from decrypted and decompressed chunks
 
 ### 5. Pruning
@@ -232,9 +232,8 @@ The restore process doesn't use the local database. Instead:
 
 Before each backup:
 1. Query incomplete snapshots (where `completed_at IS NULL`)
-2. Check if metadata exists in S3
-3. If no metadata, delete snapshot and all associations
-4. Clean up orphaned files, chunks, and blobs
+2. Delete each one and all its associations, without checking S3 for its metadata
+3. Clean up orphaned files, chunks, and blobs
 
 ## Repository Pattern