Skeleton, mnemonic input, derivation, and the ssh pub and priv commands (closes #1)

The module, the script entrypoints and Makefile, Docker-only linting, the mnemonic sources in the specified order with their refusals, the BIP-85 derivation, and keyfunc ssh pub and priv with the README test vectors as tests. Two review rounds; the second passed with no findings.

Model: opus-5 (implementation and review); fable-5-1 (landing)

.golangci.yml

@@ -0,0 +1,34 @@
+version: "2"
+
+# Config schema uses the golangci-lint v2 layout (settings live under
+# linters.settings, not top-level linters-settings) so that the
+# thresholds below are actually applied by golangci-lint >= v2.
+
+run:
+  timeout: 5m
+  modules-download-mode: readonly
+
+linters:
+  default: all
+  disable:
+    # Genuinely incompatible with project patterns
+    - exhaustruct      # Requires all struct fields
+    - depguard         # Dependency allow/block lists
+    - godot            # Requires comments to end with periods
+    - wsl              # Deprecated, replaced by wsl_v5
+    - wrapcheck        # Too verbose for internal packages
+    - varnamelen       # Short names like db, id are idiomatic Go
+  settings:
+    lll:
+      line-length: 88
+    funlen:
+      lines: 80
+      statements: 50
+    cyclop:
+      max-complexity: 15
+    dupl:
+      threshold: 100
+
+issues:
+  max-issues-per-linter: 0
+  max-same-issues: 0