Warn when a tool is defined in multiple configuration files

[ZayanKhan-12]

Closes #63

Foreman aggregates foreman.toml files from the current directory up to the filesystem root, plus the user configuration, with the first definition of a tool alias taking precedence (ConfigFile::fill_from). When two files define the same alias differently, the override was completely silent — exactly the footgun described in #63.

fill_from now logs a warning naming the alias, the definition in effect, and the one being ignored:

tool `rojo` is defined in multiple configuration files; using `https://github.com/rojo-rbx/rojo@=7.4.0` (ignoring `https://github.com/rojo-rbx/rojo@=6.2.0`)

Design notes:

• warn, not error — nested-project configs overriding an outer/user config is existing, presumably intentional behavior; erroring would break that workflow.
• identical duplicate definitions stay silent (nothing surprising is happening).
• precedence is unchanged, covered by a new test (duplicate_tool_definitions_keep_the_first_one).

Verification

• cargo test --locked: all suites pass (48 tests, including the new one)
• cargo fmt -- --check clean; cargo clippy introduces no new warnings

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