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`

AGENTS.md

@@ -322,6 +322,28 @@ High risk: all seven roles.
 - Use environment variables or vault tooling for sensitive configuration.
 - For localhost-sensitive tests, verify proxy settings to avoid traffic leakage.
 
+## Logging
+
+Applies to **every** `tracing` macro you add or edit, including a single line
+added in passing while fixing something else — not only to log-focused changes.
+
+- Fields first, message second: `event`, `component`, `subsystem`,
+  `result`/`state`, then key context. The message is a short label, not a
+  sentence with values interpolated into it.
+- Reuse the existing `EVENT_*` / `LOG_COMPONENT_*` / `LOG_SUBSYSTEM_*`
+  constants of the module you are editing; match the shape of the log sites
+  already in that file rather than introducing a second style next to them.
+- Level policy: `error` for behavior/security-affecting failures, `warn` for
+  degraded or fallback paths, `info` for low-frequency lifecycle, `debug` for
+  targeted diagnostics, `trace` for hot paths. Per-object and per-request
+  success paths are `trace`.
+- Never log secrets, tokens, credential payloads, or merged config dumps.
+- `scripts/check_logging_guardrails.sh` enforces a subset of this on the files
+  it lists; passing it is a floor, not evidence the log matches the house style.
+
+See `.agents/skills/rustfs-logging-governance/SKILL.md` for the full event
+model, level policy, and guardrail-update checklist.
+
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