feat: rename `card get` to `card inspect`

## Problem

`card get` is misleading — the verb "get" implies a remote fetch, but the command works equally well with local `file://...` references where nothing is "gotten". The command's real purpose is **displaying/inspecting the contents of an agent card**, regardless of where it lives.

## Proposal

Rename `card get` to `card inspect` (preferred) or `card show`.

**Why `inspect`:**
- Matches the mental model — you are inspecting a card's contents
- Consistent with other CLI tooling conventions (`docker inspect`, `kubectl describe`)
- Works equally well for remote URLs and local `file://` references
- Avoids confusion with "fetching" or "downloading"

## Implementation notes

- Rename the command in `internal/cli/card_get.go` (and the file itself)
- Update `card.go` subcommand registration
- Add a deprecated `get` alias pointing to `inspect` for backwards compatibility
- Update README / docs

## References

- `internal/cli/card_get.go` — command implementation
- `internal/cli/card.go` — parent command registration