misc: persist Error-message style in CONTRIBUTING.md and AGENTS.md

In [0] we agreed on the current format.

[0] https://github.com/cloud-hypervisor/cloud-hypervisor/pull/7066

On-behalf-of: SAP philipp.schuster@sap.com
Signed-off-by: Philipp Schuster <philipp.schuster@cyberus-technology.de>

AGENTS.md

@@ -31,6 +31,11 @@ reviewable, and compatible with the project's normal engineering constraints.
   migration implications.
 - Do not invent APIs, behavior, or requirements. If something is uncertain,
   state the uncertainty and proceed only with minimal, explicit assumptions.
+- For `thiserror`-style errors, start messages with a capital letter and keep
+  the outer `Display` text short. Put all non-`#[source]` attributes in the
+  message to improve helpfulness, but do not repeat a `#[source]` value
+  inline: Cloud Hypervisor prints the full error chain, so only include the
+  concrete failure text directly when there is no source to report.
 
 ### Safety and Domain Notes
 

CONTRIBUTING.md

@@ -44,6 +44,12 @@ state changes that matter in production; use `warn!` or `error!` only for
 abnormal conditions. Keep `debug!` for focused diagnostics. Please find more
 information in [`docs/logging.md`](docs/logging.md).
 
+Error messages should be sentence-style: start with a capital letter and stay
+concise. For `thiserror`-style errors, put all non-`#[source]` attributes
+(if they provide clear value) in the outer `Display` text to improve helpfulness,
+but do not repeat a `#[source]` value there because Cloud Hypervisor prints the
+full chain elsewhere.
+
 [Rust Style]: https://github.com/rust-lang/rust/tree/HEAD/src/doc/style-guide/src
 
 ## Basic Checks