Recipe sessions lose delegate tool when extensions block omits summon

[clouatre]

Describe the bug

PR #6964 moved delegate from an unconditional builtin into the summon platform extension. Recipes with an explicit extensions: block that omit summon silently lose access to delegate with no error or warning.

To Reproduce

1. Create a recipe with an explicit extensions: block that omits summon but uses delegate via inline instructions
2. Run: goose run --recipe your-recipe.yaml
3. delegate is unavailable; the agent does not spawn sub-agents

Expected behavior

delegate is available in any recipe session, consistent with pre-#6964 behavior where subagent was registered unconditionally in agent.rs regardless of the recipe's extensions: block.

Additional context

resolve_extensions_for_new_session treats a recipe's extensions: block as exclusive and does not merge with ~/.config/goose/config.yaml. If summon is absent, delegate is never registered.

Commit b01e58b16 auto-injects summon for recipes with a sub_recipes: field but does not cover recipes that invoke delegate via inline instructions.

Workaround: add summon to the recipe's extensions: block:

extensions:
  - type: platform
    name: summon
  - type: builtin
    name: developer

Proposed fixes

1. Extend ensure_summon_for_subrecipes to cover all recipes, not just those with sub_recipes:
2. Emit a warning when a recipe session starts without summon loaded
3. Add a migration note to the recipe authoring docs

[clouatre]

Fixed in #7360. ensure_summon_extension (formerly ensure_summon_for_subrecipes) now also scans instructions for \bdelegate\s*\( and auto-injects the summon extension when absent.