[vmaerten]

Summary

Customising a shared Taskfile locally today means copying the whole file over a .dist variant (see #1873). When Task discovers the root Taskfile itself, it now also loads an optional Taskfile.override.yml next to it: vars and env merge by key with the override winning, tasks are added or replaced whole, and any other root key is an error. Naming the file with --taskfile skips the override; included, remote and stdin Taskfiles never load one.

Reviewer decision: tasks are replaced whole rather than merged field by field, so #1873's own example (overriding only a task's env) needs the value moved to a variable. The JSON schema is not covered yet: editors won't recognise Taskfile.override.yml until SchemaStore maps it, and the main schema would accept keys the override rejects.

Test plan

• TestAutomaticOverride (golden, testdata/override): discovery, subdirectory with a stray invalid override, -t file vs -t directory, TASKFILE/ROOT_TASKFILE for an added task.
• taskfile/override_test.go: merge semantics and ordering, override name priority, decode/version/forbidden-key errors, unreadable override, override applied before includes and conflict with an included task, remote and stdin roots ignored.
• TestAutomaticOverrideGlobal: --global loads the override from the home directory.
• Manual: check the new "Customize a Taskfile locally" section renders correctly on the docs site.