Fix clippy warnings in external agent config migration

[canvrno-oai]

Fix clippy warnings in external agent config migration

error: this expression creates a reference which is immediately dereferenced by the compiler
   --> core/src/external_agent_config.rs:188:55
    |
188 |             let migrated = build_config_from_external(&settings)?;
    |                                                       ^^^^^^^^^ help: change this to: `settings`
    |
    = help: for further information visit https://rust-lang.github.io/rust-clippy/rust-1.93.0/index.html#needless_borrow
    = note: requested on the command line with `-D clippy::needless-borrow`

error: useless conversion to the same type: `codex_utils_absolute_path::AbsolutePathBuf`
   --> core/src/external_agent_config.rs:355:27
    |
355 |                       match AbsolutePathBuf::try_from(
    |  ___________________________^
356 | |                         add_marketplace_outcome
357 | |                             .installed_root
358 | |                             .join(INSTALLED_MARKETPLACE_MANIFEST_RELATIVE_PATH),
359 | |                     ) {
    | |_____________________^
    |
    = help: consider removing `AbsolutePathBuf::try_from()`
    = help: for further information visit https://rust-lang.github.io/rust-clippy/rust-1.93.0/index.html#useless_conversion
    = note: `-D clippy::useless-conversion` implied by `-D warnings`
    = help: to override `-D warnings` add `#[allow(clippy::useless_conversion)]`

error: aborting due to 2 previous errors