Add Markdown index HTML output mode

docs/internal/config.md

@@ -24,6 +24,7 @@ Defaults are applied in `ApplyDefaults`:
 - SSH backend `port` defaults to `22`;
 - SSH backend `host_key_policy` defaults to `accept-new`;
 - destination publish policy defaults to source output only;
+- Markdown-to-HTML mode defaults to `sidecar` when a transform block is present and mode is omitted;
 - `transfer.on_destination_same` defaults to `skip`;
 - `transfer.on_destination_older` defaults to `replace`;
 - `transfer.on_destination_newer` defaults to `skip`;
@@ -35,7 +36,7 @@ Validation requires at least one pipeline, slug-like unique pipeline ids, one so
 
 Transfer validation accepts `replace` for `on_destination_newer` and `on_conflict`, but publish planning honors those destructive actions only when the current run explicitly requests force.
 
-`ValidatePublishTransformPolicy` is shared with publish planning so destination policy combinations are checked consistently. Publishing HTML requires an enabled Markdown-to-HTML transform in `sidecar` mode. A publish policy must select source output, HTML output, or both.
+`ValidatePublishTransformPolicy` is shared with publish planning so destination policy combinations are checked consistently. Publishing HTML requires an enabled Markdown-to-HTML transform in `sidecar` or `index` mode. Enabled Markdown-to-HTML config is rejected when `publish.html` is false. `input` is accepted only for enabled `index` mode. A publish policy must select source output, HTML output, or both.
 
 ## Executable support boundary
 

docs/internal/publish.md

@@ -22,7 +22,7 @@ Execution fails if a write, delete, state serialization, or context check fails.
 
 ## Boundaries
 
-The current implementation publishes source files and Markdown-to-HTML sidecar outputs. Backend behavior is supplied through `internal/storage`; app runtime currently supplies local, SSH, and S3 backends.
+The current implementation publishes source files and Markdown-to-HTML outputs. Markdown sidecar mode writes same-directory `.html` outputs, and Markdown index mode writes `index.html`. Backend behavior is supplied through `internal/storage`; app runtime currently supplies local, SSH, and S3 backends.
 
 The package uses `internal/state` for destination comparison, `internal/storage` for IO, and the shared `internal/config` publish/transform policy helper for request validation. It resolves transforms through a narrow resolver supplied by the caller; concrete transform registration is owned by the app layer. It does not parse CLI flags or load config files.
 

docs/internal/transform.md

@@ -2,11 +2,11 @@
 
 ## Purpose
 
-`internal/transform` defines generated publication artifacts. `internal/transform/markdown` implements Markdown-to-HTML sidecar generation.
+`internal/transform` defines generated publication artifacts. `internal/transform/markdown` implements Markdown-to-HTML generation.
 
 ## Inputs and outputs
 
-Inputs are a validated source bundle and source backend. Outputs include destination path, source path, transform id, generated bytes, SHA-256, and size.
+Inputs are a validated source bundle, source backend, and transform options supplied by publish planning. Outputs include destination path, source path, transform id, generated bytes, SHA-256, and size.
 
 ## Registry
 
@@ -14,7 +14,7 @@ Inputs are a validated source bundle and source backend. Outputs include destina
 
 ## Markdown behavior
 
-Markdown files ending in `.md` generate `.html` files in the same logical directory. Non-Markdown files do not generate outputs. Raw HTML embedded in Markdown is not passed through by the renderer.
+Markdown sidecar mode renders files ending in `.md` to `.html` files in the same logical directory. Markdown index mode renders one selected manifest-listed Markdown file to `index.html`. Non-Markdown files do not generate sidecar outputs. Raw HTML embedded in Markdown is not passed through by the renderer.
 
 Generated HTML is deterministic for the same source content and transform configuration.
 
@@ -22,7 +22,7 @@ See `docs/integrations/markdown.md` for the Goldmark integration contract.
 
 ## Failure behavior
 
-Transform resolution fails when a requested transform id is not registered. Markdown rendering fails when the source file cannot be read or rendered. Publish planning fails when HTML output is requested and the selected transform produces no outputs for a bundle.
+Transform resolution fails when a requested transform id is not registered. Markdown rendering fails when the source file cannot be read or rendered. Index input selection fails when the configured input is unsafe, not listed, not Markdown, or when no configured input can be inferred from exactly one manifest-listed Markdown file. Publish planning fails when HTML output is requested and the selected transform produces no outputs for a bundle.
 
 ## Boundaries
 
@@ -42,5 +42,6 @@ Before changing transform behavior, inspect tests under:
 - Source bundle files are never mutated by transforms.
 - Generated outputs record destination path, source path, transform id, SHA-256, and size.
 - Markdown sidecar naming changes only the `.md` extension to `.html`.
+- Markdown index mode always writes `index.html`.
 - Non-Markdown source files do not generate Markdown outputs.
 - Transform registration stays outside publish planning.