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/ok-block-scalars/content/about/background.md

@@ -0,0 +1,25 @@
+---
+# a leading comment is allowed
+id: background
+area: about
+level: 1
+title: >-
+  Twenty-five years of delivery
+claim: |
+  Shipped software since 2000.
+  Now agent systems and the platform under them.
+confidence: delivered
+period: "2000-"
+tech:
+  - "NixOS"
+  - 'Golang'
+metrics:
+  - label: years
+    value: 25
+artefacts: []
+public: true
+last_verified: 2026-10-20
+tags: [narrative, career]
+---
+
+# Twenty-five years of delivery