[codex] Add cross-repo plugin sources to marketplace manifests (#18017)

## Summary
- add first-class marketplace support for git-backed plugin sources
- keep the newer marketplace parsing behavior from `main`, including
alternate manifest locations and string local sources
- materialize remote plugin sources during install, detail reads, and
non-curated cache refresh
- expose git plugin source metadata through the app-server protocol

## Details
This teaches the marketplace parser to accept all of the following:
- local string sources such as `"source": "./plugins/foo"`
- local object sources such as
`{"source":"local","path":"./plugins/foo"}`
- remote repo-root sources such as
`{"source":"url","url":"https://github.com/org/repo.git"}`
- remote subdir sources such as
`{"source":"git-subdir","url":"owner/repo","path":"plugins/foo","ref":"main","sha":"..."}`

It also preserves the newer tolerant behavior from `main`: invalid or
unsupported plugin entries are skipped instead of breaking the whole
marketplace.

## Validation
- `cargo test -p codex-core plugins::marketplace::tests`
- `just fix -p codex-core`
- `just fmt`

## Notes
- A full `cargo test -p codex-core` run still hit unrelated existing
failures in agent and multi-agent tests during this session; the
marketplace-focused suite passed after the rebase resolution.

codex-rs/app-server-protocol/schema/json/v2/PluginReadResponse.json

@@ -224,6 +224,44 @@
           ],
           "title": "LocalPluginSource",
           "type": "object"
+        },
+        {
+          "properties": {
+            "path": {
+              "type": [
+                "string",
+                "null"
+              ]
+            },
+            "refName": {
+              "type": [
+                "string",
+                "null"
+              ]
+            },
+            "sha": {
+              "type": [
+                "string",
+                "null"
+              ]
+            },
+            "type": {
+              "enum": [
+                "git"
+              ],
+              "title": "GitPluginSourceType",
+              "type": "string"
+            },
+            "url": {
+              "type": "string"
+            }
+          },
+          "required": [
+            "type",
+            "url"
+          ],
+          "title": "GitPluginSource",
+          "type": "object"
         }
       ]
     },