fix(agents): honor GOOSE_PATH_ROOT for Goose MCP

[Fluzko]

What does this PR do?

Part of #277.
Depends on #349 and #333 (this branch carries #333's commit; until they merge, only the last commit is new here).

Goose reads its config.yaml from $GOOSE_PATH_ROOT/config/ when that variable is set, ignoring ~/.config/goose/ and XDG_CONFIG_HOME. Symposium still wrote its MCP servers to ~/.config/goose/config.yaml, so for anyone who relocated Goose they never loaded, while the plugins from #349 already went to $GOOSE_PATH_ROOT.

MCP servers now go to $GOOSE_PATH_ROOT/config/config.yaml too. Without the variable, or with it empty, nothing changes.

Verified against Goose 1.54.0 with GOOSE_PATH_ROOT set: Goose started symposium's MCP server and listed the plugin's skill from the relocated directory, with nothing written under the home directory.

Disclosure questions

AI disclosure.

• The AI tool authored large parts of the code

[Fluzko]

Folded into #333.