Quote a string setting that YAML would read as a number (closes #229)

config set wrote every value as an unquoted YAML scalar, and config.Load
reads the file through untyped YAML, so an access key 00112233 loaded as
38043 and a hostname 007 as 7.

config set now looks the key up in config.Config by the fields' yaml
tags. A string setting is tagged !!str, which the encoder quotes
wherever YAML would read a number or a boolean. Other settings stay
unquoted, so compression_level 9 is still a number. A value that is not
valid UTF-8 stays untagged: the encoder refuses it as !!str and writes
it as !!binary, which loads back unchanged.

Judgement call: the type comes from reflection over config.Config.
Quoting every value YAML would change avoids that, but would also quote
odd numbers such as compression_level 03 and make them fail to load.

Model: opus-5-5

TODO.md

@@ -22,6 +22,14 @@ the tag exists and is exercised; what is left is merging `next` to
 
 # Completed Steps
 
+- 2026-10-07: Made `config set` keep a string that looks like a number
+  ([issue #229](https://git.eeqj.de/sneak/vaultik/issues/229)). It wrote
+  every value unquoted, and `config.Load` reads the file through untyped
+  YAML, so an access key `00112233` loaded as `38043` and a hostname `007`
+  as `7`. A value for a string setting in `config.Config` is now tagged as
+  a YAML string, which the file quotes wherever YAML would read a number or
+  a boolean; other settings are still written unquoted.
+
 - 2026-10-07: Made a backup notice a file rewritten with its size
   unchanged and a new mtime in the same second as the one in the index
   ([issue #226](https://git.eeqj.de/sneak/vaultik/issues/226)). The