Phase 1a: content schema validator (freeze the schema)

Implements AGENTS.md section 7 front-matter schema as a single-file,
standard-library-only Python validator; no third-party YAML dependency
and no framework. Adds the `make validate` gate, a 20-case fixture set
and docs/content-schema.md.

DoD satisfied:
- make validate runs and exits 0 on an empty/clean content/ tree
  (missing dir and empty dir both exit 0; fixture ok-empty covers it).
- Every violation class has a fixture under test/fixtures/fail-*; each
  exits non-zero with one line per violation (file:line: field: rule).
  `make test` runs all 20 cases: 20/20 pass.
- Dangling artefact reference is fatal (fail-dangling-artefact;
  artefacts[].url must resolve to an existing in-repo L3 stub or asset).
- Unknown front-matter field is fatal (fail-unknown-field), including
  unknown keys nested inside artefacts[]/metrics[].
- docs/content-schema.md committed; states the frozen schema, links to
  AGENTS.md section 7, and records that any later schema change is a
  breaking change requiring Sam's approval.
- Scope respected: no content authored, no rendering, no site code.

Files: Makefile, tools/validate_content.py, test/fixtures/**,
docs/content-schema.md, .gitignore (__pycache__).

test/fixtures/fail-level-enum/content/assets/audio-rack.svg

@@ -0,0 +1 @@
+<svg xmlns="http://www.w3.org/2000/svg" width="10" height="10"/>

test/fixtures/fail-level-enum/content/edge/audio.md

@@ -0,0 +1,21 @@
+---
+id: audio
+area: edge
+level: 4
+title: "Distributed Audio"
+claim: "Eight synchronised endpoints with music ducking for voice."
+confidence: running
+period: "2023-"
+tech: [Snapcast, Mopidy, Spotify]
+metrics: [{label: "endpoints", value: 8}]
+artefacts:
+  - type: doc
+    label: "Architecture note"
+    url: "content/edge/l3/audio-architecture.md"
+  - {type: image, label: "Rack diagram", url: "content/assets/audio-rack.svg"}
+public: true
+last_verified: 2026-10-20
+tags: [audio, snapcast]
+---
+
+# Distributed Audio

test/fixtures/fail-level-enum/content/edge/l3/audio-architecture.md

@@ -0,0 +1,17 @@
+---
+id: audio-architecture
+area: edge
+level: 3
+title: "Audio architecture note"
+claim: "How the eight endpoints stay in sync."
+confidence: running
+period: "2023-"
+tech: []
+metrics: []
+artefacts: []
+public: true
+last_verified: 2026-10-20
+tags: [audio]
+---
+
+# Audio architecture note

test/fixtures/fail-level-enum/expect.txt

@@ -0,0 +1 @@
+level: must be the integer 1, 2 or 3