fix: remove doubled prefixes from min CLI error output - #1151

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Reviewed — the shared-type change is safe, and it improves an existing message

The risky part of this diff is changing mfile::Error::NotFound's Display from "{MFILE_NAME} not found" to bare "not found". That is a shared error type, so any caller rendering it standalone would lose the filename and print an unhelpful bare not found.

I checked every consumer. All of them pattern-match the variant and construct their own message — decode/src/lib.rs:145, mctx/src/project_setup.rs:36, mfile/src/lib.rs:802/809/814, minimal/src/lib.rs:1423/1492, minimal/src/task.rs:96, minimald/src/session.rs:1451, minimald/src/sessions/composables.rs:265. None relies on the old Display text.

The one place that does render it is minimal/src/lib.rs:2846 — the min init hint added in #1140, which formats "{e}\nRun 'min init'...". That path goes through mctx::Error::MFile, whose Display supplies the filename, so it now reads:

minimal.toml: not found
Run 'min init' to give the project its own config.

Before this change it produced minimal.toml: minimal.toml not found — the exact doubling this PR exists to fix. So #1140's hint gets better, not worse.

The map_err(|e| anyhow::anyhow!("{e}")) flatten in resolve_active_loadouts is the right shape too: LoadError's Display already embeds its I/O source, so collapsing it to a leaf before .with_context is what stops anyhow re-rendering the chain. Both regression tests assert the single-prefix form directly rather than asserting on a substring, which is what makes them worth having.