feat(cli)!: replace upgrade and add update checks

CLAUDE.md

@@ -24,7 +24,7 @@ cargo clippy --workspace --all-targets -- -D warnings
 
 ## Product boundary
 
-The native `skilld` CLI searches, installs, lists, views, removes, upgrades, and verifies Skills.
+The native `skilld` CLI searches, installs, lists, views, removes, updates, and verifies Skills.
 It also manages account authentication and Agent target configuration.
 
 The skilld CLI contains no Skill generation logic or Agent runtime.

GLOSSARY.md

@@ -19,6 +19,7 @@ Every public export, command, error, route, and document uses these terms.
 | Artifact attestation | `skilld.dev/api/v1` | published protocol | skilld CLI | attestation |
 | Check result | `skilld.dev/api/v1` | published protocol | skilld CLI, developer | check result |
 | Source status | lockfile and protocol | published value | skilld CLI, CI | source status |
+| Update relation | skilld CLI JSON v1 | published value | Agent, developer, CI | update relation |
 | Agent target | skilld CLI | published configuration | Agent | Agent target |
 
 | Identifier | Term |
@@ -28,7 +29,8 @@ Every public export, command, error, route, and document uses these terms.
 | `skilld list` | installed Skills |
 | `skilld view` | Skill details |
 | `skilld remove` | Skill removal |
-| `skilld upgrade` | Skill upgrade |
+| `skilld update` | Skill update |
+| `skilld update --check --json` | update relation check |
 | `skilld verify` | source verification |
 | `skilld install skilld --global` | global skilld Skill install |
 | `skilld auth login` | account login |
@@ -94,7 +96,7 @@ None recorded.
 
 ### skilld CLI
 
-**Is:** the Rust command line interface that searches, installs, upgrades, and removes Skills.
+**Is:** the Rust command line interface that searches, installs, updates, and removes Skills.
 
 **Use for:** the command product and its manager logic.
 
@@ -182,6 +184,16 @@ None recorded.
 
 **Casing:** `Source status` in headings, `sourceStatus` in identifiers.
 
+### Update relation
+
+**Is:** the Git relationship between an installed Skill commit and its current source commit.
+
+**Use for:** `current`, `available`, `behind`, `diverged`, `pinned`, `notTracked`, or `unavailable` JSON values.
+
+**Never:** upgrade status, version status, release status.
+
+**Casing:** `Update relation` in headings, `relation` in JSON.
+
 ### Agent target
 
 **Is:** an Agent installation destination managed by skilld.

README.md

@@ -49,7 +49,10 @@ skilld view vue
 
 # Keep Skills current
 skilld verify vue
-skilld upgrade vue
+skilld update vue
+
+# Check update relations for an Agent or CI
+skilld update --check --json
 
 # Remove a Skill
 skilld remove vue