Centralize agent instructions via nested includes

CLAUDE.md includes docs/development/index.md. AGENTS.md and
copilot-instructions.md include CLAUDE.md, getting everything
transitively via recursive include expansion.

Extract coverage gate and file placement into separate files
to keep the dev index short (CLAUDE.md stays under 300 lines).
Use static links in the dev index pending #133 (catalog glob
context fix for nested includes).

https://claude.ai/code/session_01J3g8NnEwGYyJWXD1bqtTii

Author: claude

.github/copilot-instructions.md

@@ -135,17 +135,9 @@ heading-level: "absolute"
 Build and test reference for mdsmith contributors.
 See also:
 
-<?catalog
-glob:
-  - "*.md"
-  - "!index.md"
-sort: path
-row: "- [{summary}](../docs/development/{filename})"
-?>
-- [Codecov coverage gate and CI status checks.](../docs/development/coverage.md)
-- [Where to place Markdown files and documentation types.](../docs/development/file-placement.md)
-- [Rebase, CI monitoring, and review comment resolution.](../docs/development/pr-fixup-workflow.md)
-<?/catalog?>
+- [Coverage gate](../docs/development/coverage.md)
+- [File placement](../docs/development/file-placement.md)
+- [PR fixup workflow](../docs/development/pr-fixup-workflow.md)
 
 #### Build & Test Commands
 

AGENTS.md

@@ -142,17 +142,9 @@ heading-level: "absolute"
 Build and test reference for mdsmith contributors.
 See also:
 
-<?catalog
-glob:
-  - "*.md"
-  - "!index.md"
-sort: path
-row: "- [{summary}](docs/development/{filename})"
-?>
-- [Codecov coverage gate and CI status checks.](docs/development/coverage.md)
-- [Where to place Markdown files and documentation types.](docs/development/file-placement.md)
-- [Rebase, CI monitoring, and review comment resolution.](docs/development/pr-fixup-workflow.md)
-<?/catalog?>
+- [Coverage gate](docs/development/coverage.md)
+- [File placement](docs/development/file-placement.md)
+- [PR fixup workflow](docs/development/pr-fixup-workflow.md)
 
 ### Build & Test Commands
 

CLAUDE.md

@@ -128,17 +128,9 @@ heading-level: "absolute"
 Build and test reference for mdsmith contributors.
 See also:
 
-<?catalog
-glob:
-  - "*.md"
-  - "!index.md"
-sort: path
-row: "- [{summary}](docs/development/{filename})"
-?>
-- [Codecov coverage gate and CI status checks.](docs/development/coverage.md)
-- [Where to place Markdown files and documentation types.](docs/development/file-placement.md)
-- [Rebase, CI monitoring, and review comment resolution.](docs/development/pr-fixup-workflow.md)
-<?/catalog?>
+- [Coverage gate](docs/development/coverage.md)
+- [File placement](docs/development/file-placement.md)
+- [PR fixup workflow](docs/development/pr-fixup-workflow.md)
 
 ### Build & Test Commands