docs(adr): configUrl (primary) and configToml (inline/--set-file) over Helm rendering (#1271)

* docs(adr): configUrl as primary config path over Helm rendering

* docs(adr): highlight Helm's 3 key values — non-root, readonly fs, HOME PVC

* docs(adr): add configFile mode — local config.toml as verbatim ConfigMap

* docs(adr): mark legacy values.yaml rendering as deprecated and unmaintained

* docs(adr): fix author to @chaodu-agent

* docs(adr): emphasize configUrl as platform-agnostic primary path

- Add section 3a explaining why configUrl is the primary path, not
  just 'the production option': platform parity across K8s/ECS/
  Zeabur/AgentCore, fleet-scale 1:N shared config with hot-reload-
  by-restart, and config as a shareable credential-free artifact.
- Reframe configFile/configToml as secondary, Kubernetes-only
  conveniences for local iteration rather than peers of configUrl.
- Update the configuration modes table to show platform scope per
  mode and mark configUrl as primary / configFile as secondary.
- Update consequences (positive) to call out platform parity,
  fleet hot-reload, and shareable config as first-class benefits.

Kubernetes should not dictate config architecture for users on
ECS, Zeabur, or AgentCore Runtime who have no reason to adopt Helm.
configUrl gives every platform the same low-friction workflow.

* docs(adr): document --set-file for configToml as-is file loading

- Correct section 3a/4/5 references from a separate configFile field
  (.Files.Get) to configToml's actual dual usage: inline TOML string
  or --set-file to load an external config.toml verbatim.
- Add --set-file example showing WYSIWYG standalone-file editing
  without any chart changes, addressing the request for users to
  keep config.toml as-is in its own file with IDE spec checking.
- Clarify no dedicated configFile field is needed since --set-file
  already covers that use case via the existing configToml field.

* docs(adr): clarify configToml dependency direction vs #1276

Addresses review finding F1: the ADR previously stated configToml
was 'implemented in #1276' as if #1276 were already merged, but
#1276 is the PR this ADR's decision motivates (dependency direction
was backwards). Added an Implementation note in the header stating
merge order explicitly: #1276 -> this ADR -> #1277.

F2 (dangling doc links: docs/config-reference.md,
docs/secrets-management.md, operator/examples/fleet.yaml) was
checked and all three files exist on main -- no fix needed, likely
a stale check against the PR diff rather than the full repo tree.

---------

Co-authored-by: 超渡法師 <chaodu-agent@openab.dev>
Co-authored-by: chaodu-agent <chaodu-agent@users.noreply.github.com>

Author: chaodu-agent