revert(lint): back out the canonical .golangci.yml adoption — repo has a nested module

Adopting it turned CI red, and my pre-push verification missed why:
`golangci-lint run ./...` does NOT traverse into nested Go modules. This repo
has two — ./go.mod and ./ingestion/go.mod. I linted the root (0 issues, which
CI reproduces exactly) and never linted ingestion/, which has 3 findings:

  ingestion/internal/audit/degrading.go:81   missing case capture.AuditCentral   (exhaustive)
  ingestion/internal/capture/entities.go:37  missing case capture.ZoneUnknown    (exhaustive)
  ingestion/internal/mcp/handlers.go:29      var int_ should be int              (revive)

Those are behavioural questions and a rename, not formatting — the same class
of finding that kept cad, dispatch, ingestion-svc, dma-cli and
email-triage-agent out of the earlier waves. Consistent with that, this repo
goes back in the queue rather than being force-fitted now.

The gofumpt formatting from the same commit is kept: it is independently
correct and already green.

Verification method corrected going forward: a repo with nested modules must
be linted per module, not once from the root.

.golangci.yml

@@ -1,52 +0,0 @@
-# Canonical golangci-lint configuration for the estate's Go repos (infra#309
-# pattern: one canonical file here, a byte-identical copy in each adopter,
-# and the drift gate in THIS repo comparing sha256).
-#
-# Why this exists: 26 Go repos, 3 had a lint config, and all three were
-# different. The 23 without one ran golangci-lint's v2 default set, which
-# does NOT include a formatter -- so gofmt/gofumpt drift went uncaught until
-# it was found by hand (dma-cli#12: two files drifted, nothing flagged them).
-#
-# The two configs this replaces (cobalt-dingo, coo-agent) were near-identical
-# copies of each other AND both invalid under v2: they carried v1-style
-# `linters-settings` and `issues.exclude-rules` keys, which `golangci-lint
-# config verify` rejects outright. Their revive settings and test exclusions
-# were therefore not in effect. Consolidating fixes that too.
-#
-# Per-repo variation does NOT belong here -- this file must stay byte-identical
-# across adopters so the gate is a checksum, not a diff a human has to judge.
-# A repo with a genuine exception (agentsquad excludes its seeded-bug fixture
-# tree, which exists precisely to contain broken code) keeps its own config and
-# is not listed as an adopter.
-
-version: "2"
-
-linters:
-  default: none
-  enable:
-    - errcheck # unchecked errors
-    - govet # suspicious constructs
-    - staticcheck # broad static analysis
-    - unused # unused code
-    - misspell # spelling
-    - revive # idiomatic Go
-    - exhaustive # exhaustive switch on enums
-
-  settings:
-    revive:
-      rules:
-        - name: var-naming
-          severity: warning
-
-  exclusions:
-    rules:
-      # Test files: table-driven cases legitimately carry unused struct fields
-      # and long functions.
-      - path: _test\.go
-        linters:
-          - revive
-          - unused
-
-formatters:
-  enable:
-    - gofumpt