platforms: add the three platform guides the /platforms/ page renders

The /platforms/ page (408d6cd) reads platforms/*.yml and fails the build when
they are absent. Those three files were introduced on feat/phase-4-ingestion by
the ingestion commit 1e46ec0 and nothing else on the site needed them, so the
cherry-pick of 408d6cd left the tree unbuildable.

They are data, not ingestion code: each file is config plus public copy that the
site shows verbatim so a visitor can see exactly what the assistant reads. No
corpus/, no harvester, no Prefect flow and no Qdrant code is included.

Taken verbatim from 1e46ec0.

go test ./...: pass.

platforms/gitea.yml

@@ -0,0 +1,46 @@
+# Platform guide — Gitea.
+#
+# This file is both configuration and public copy. The site shows it so a
+# visitor can see exactly what the assistant reads. Keep the wording honest.
+name: gitea
+enabled: true
+what: >-
+  Self-hosted Git. The source of truth for every current project: README files,
+  file trees, selected source files, recent commit messages and language
+  statistics. This is where the work itself lives.
+fits: [ai, platform, edge, products]
+base: https://gitea.lab.audasmedia.com.au
+auth: GITEA_TOKEN
+include:
+  repos: all
+  # No path include means: every text file the repository rules admit.
+  paths: []
+exclude:
+  repos:
+    # The vault is read by the obsidian guide, so its notes are not harvested
+    # twice under a different platform.
+    - sam/obsidian-vault
+  paths:
+    - "**/node_modules/**"
+    - "**/.venv/**"
+    - "**/venv/**"
+    - "**/.git/**"
+    - "**/.trash/**"
+    - "**/.obsidian/**"
+    - "**/.pi/**"
+    - "**/__pycache__/**"
+    - "**/graphify-out/**"
+    - "**/graphify-out-temp/**"
+    - "**/vendor/**"
+    - "**/third_party/**"
+    - "**/*.min.js"
+    - "**/*.min.css"
+    - "**/*.lock"
+read: [readme, tree, selected_source, commits, languages]
+public: true
+limits:
+  max_documents: 4000
+  max_file_kb: 200
+  max_files_per_repo: 40
+  max_commits: 20
+  max_tree_entries: 3000