[chore](trx-app): remove plugin system

Drop plugins.rs module and its sha2/hex/libc dependencies.
Plugin system was not part of the codebase — mark P0 and P3
plugin items as dropped in improvement areas doc.

https://claude.ai/code/session_01Gj1vEkP6GKVcVaMqzFW885
Signed-off-by: Claude <noreply@anthropic.com>

Cargo.lock

@@ -2428,17 +2428,6 @@ dependencies = [
  "digest",
 ]
 
-[[package]]
-name = "sha2"
-version = "0.10.9"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "a7507d819769d01a365ab707794a4084392c824f54a7a6a7862f8c3d0892b283"
-dependencies = [
- "cfg-if",
- "cpufeatures",
- "digest",
-]
-
 [[package]]
 name = "sharded-slab"
 version = "0.1.7"
@@ -2975,10 +2964,7 @@ name = "trx-app"
 version = "0.1.0"
 dependencies = [
  "dirs",
- "hex",
- "libc",
  "serde",
- "sha2",
  "thiserror 2.0.17",
  "toml",
  "tracing",