feat: Cache misses are reported with a reason

When skaffold build or skaffold dev decides to rebuild an artifact, the only output is:

Checking cache...
 - app: Not found. Building

That line is the same whether:

• a source file the artifact depends on changed,
• a build arg or the artifact's config changed,
• or nothing changed at all and the previously built image was simply pruned from the local daemon or evicted from the registry.

The last case is the confusing one: nothing the user did caused it, but the output is identical to a real source change. Diagnosing an unexpected rebuild today means building a patched skaffold and printing the hash inputs by hand.

Two things would help, and I have both working locally:

1. Report the reason on the existing line. The lookup already knows whether the artifact's current hash had an entry in the cache, which separates "the inputs produced a hash we have never built" from "we built this, but the image is gone":

   复制

    - app: Not found. Building (no cached build for the current inputs)
    - api: Not found. Building (cached image is no longer available)

   No new flags, files or I/O — it is derived from state the lookup already has.

2. Behind an opt-in flag, record the inputs that feed each artifact's hash and diff them against the previous run, so the miss names what changed:

   复制

    - app: Not found. Building (inputs changed)
        ~ src/handler.go
        + src/middleware.go
        - src/old_handler.go