docs(agents): make the structured-logging rule reachable and enforceable (#5828)

The RustFS event shape (`event`/`component`/`subsystem`/`result` + context,
message last) is specified only in
`.agents/skills/rustfs-logging-governance/SKILL.md`, and nothing routes a
change to it:

- `AGENTS.md`, which is what an agent actually loads by default, never
  mentions logging. Its only related line is "log unknown fields at `warn`"
  under Serde Safety, which is about level, not shape.
- The skill's `description` says "use when editing or reviewing RustFS logs",
  so a bugfix that adds one log line in passing — how most new log sites enter
  this repo — never matches it.
- `scripts/check_logging_guardrails.sh` is a blocklist: 500+ `rg -F` literals
  that retire log lines which already shipped. It cannot see a newly written
  one. For `crates/ecstore/src/disk/local.rs` the only check is that
  `#[tracing::instrument]` is TRACE-only; `warn!`/`info!` shape is unchecked.

PR #5822 landed `warn!("heal rename_data: purging ... {:?} failed: {}", ...)`
in `disk/local.rs` — sentence-style, no fields, directly beside `info!(event =
EVENT_DISK_LOCAL_RENAME_REJECTED, component = ..., subsystem = ...)` — with
every check green. That is the gap, not an authoring mistake.

Close all three:

- `AGENTS.md`: a Logging section stating the field shape, the level policy,
  the reuse-the-file's-constants rule, and that it applies to any `tracing`
  macro added in passing, not only to log-focused changes.
- Skill `description`: trigger on adding or editing any `tracing` macro,
  naming the single-line-added-in-passing case explicitly.
- Guardrail: assert the event shape positively on the already-governed disk
  files — `error!`/`warn!`/`info!` must open with fields or a `target:`, never
  a bare string. Commented-out macros are excluded; `debug!`/`trace!` stay out
  of scope as targeted diagnostics. Self-test fixtures cover both directions.

`crates/ecstore/src/disk/mod.rs` carried the one live violation in that file
set (`conv_part_err_to_int`), so it is converted here; the guardrail would
otherwise fail on an untouched file.

Verification:
- `./scripts/check_logging_guardrails.sh` — passes
- Negative control: re-inserting PR #5822's exact `warn!` line into
  `disk/local.rs` makes it exit 1 pointing at that line
- `cargo fmt -p rustfs-ecstore -- --check`, `cargo check -p rustfs-ecstore`

crates/ecstore/src/disk/mod.rs

@@ -42,6 +42,10 @@ pub const PART_TRANSACTION_NEW_META: &str = "new.meta";
 pub const PART_TRANSACTION_OLD_META: &str = "old.meta";
 pub const PART_TRANSACTION_ROLLBACK: &str = "rollback";
 
+const LOG_COMPONENT_ECSTORE: &str = "ecstore";
+const LOG_SUBSYSTEM_DISK: &str = "disk";
+const EVENT_DISK_PART_ERR_UNCLASSIFIED: &str = "disk_part_err_unclassified";
+
 pub fn part_transaction_path(part_path: &str) -> String {
     match part_path.rsplit_once('/') {
         Some((parent, name)) => format!("{parent}/.{name}.rustfs-txn"),
@@ -1196,7 +1200,13 @@ pub fn conv_part_err_to_int(err: &Option<Error>) -> usize {
         Some(DiskError::DiskNotFound) => CHECK_PART_DISK_NOT_FOUND,
         None => CHECK_PART_SUCCESS,
         _ => {
-            tracing::warn!("conv_part_err_to_int: unknown error: {err:?}");
+            tracing::warn!(
+                event = EVENT_DISK_PART_ERR_UNCLASSIFIED,
+                component = LOG_COMPONENT_ECSTORE,
+                subsystem = LOG_SUBSYSTEM_DISK,
+                error = ?err,
+                "Part error has no check-part code and degrades to unknown"
+            );
             CHECK_PART_UNKNOWN
         }
     }