feat(runner): caching / incremental runs

[CalvinAllen]

Description

Skip tasks if inputs haven't changed since last successful run.

Proposed Syntax

build:
  inputs:
    - src/**/*.rs
    - Cargo.toml
  outputs:
    - target/release/myapp
  cmd: cargo build --release

Acceptance Criteria

• Hash input files to determine if changed
• Store cache in .rnr/cache/
• Skip task if inputs unchanged and outputs exist
• --no-cache flag to force re-run
• rnr clean-cache command

Notes

Similar to Nx, Turborepo, or Gradle build caching.