fix(hook): use per-alias content hashing for fine-grained change detection

- add compute_short_hash() to trust.rs (7-char blake3 prefix)
- change _AM_PROJECT_ALIASES format from "b,t" to "b|hash,t|hash"
- rewrite hook change detection to diff at individual alias level:
  removed = unload, added = load, changed = unload+reload, unchanged = skip
- show incremental message ("1 added, 2 updated") on alias changes
  instead of full load message (which is kept for fresh cd-into-project)
- backward compat: old format entries (no |) trigger a full reload
- add regression test for overwriting an alias value (the original bug)
- update snapshot tests for new env var format

Author: sassman

crates/am/src/env_vars.rs

@@ -2,8 +2,10 @@
 /// Value: comma-separated list, e.g. `"gs,ll"`.
 pub const AM_ALIASES: &str = "_AM_ALIASES";
 
-/// Tracks project-level alias names loaded by the cd hook.
-/// Value: comma-separated list, e.g. `"b,t"`.
+/// Tracks project-level aliases loaded by the cd hook.
+/// Value: comma-separated entries of `name|short_hash`, e.g. `"b|a132b21,t|1241ab1"`.
+/// The short hash is the first 7 hex chars of the BLAKE3 hash of the alias value,
+/// enabling per-alias change detection on reload.
 pub const AM_PROJECT_ALIASES: &str = "_AM_PROJECT_ALIASES";
 
 /// Path of the `.aliases` file currently in scope, used to suppress

crates/am/src/hook.rs

@@ -47,6 +47,14 @@ pub fn compute_hash(content: &[u8]) -> String {
     blake3::hash(content).to_hex().to_string()
 }
 
+/// Compute a 7-character short BLAKE3 hash of byte content.
+///
+/// Returns the first 7 hex characters of the BLAKE3 hash — analogous to
+/// git's short SHA. 28 bits gives collision probability <0.2 % for 1 000 items.
+pub fn compute_short_hash(content: &[u8]) -> String {
+    blake3::hash(content).to_hex()[..7].to_string()
+}
+
 /// Render the "loaded" info message shown on cd into a trusted directory.
 ///
 /// Alias names are right-padded so `->` and commands align in columns.
@@ -118,6 +126,27 @@ mod tests {
         assert_eq!(hash1, hash2);
     }
 
+    #[test]
+    fn compute_short_hash_returns_7_chars() {
+        let hash = compute_short_hash(b"make build");
+        assert_eq!(hash.len(), 7);
+        assert!(hash.chars().all(|c| c.is_ascii_hexdigit()));
+    }
+
+    #[test]
+    fn compute_short_hash_deterministic() {
+        let h1 = compute_short_hash(b"make build");
+        let h2 = compute_short_hash(b"make build");
+        assert_eq!(h1, h2);
+    }
+
+    #[test]
+    fn compute_short_hash_different_content() {
+        let h1 = compute_short_hash(b"make build");
+        let h2 = compute_short_hash(b"cargo build");
+        assert_ne!(h1, h2);
+    }
+
     #[test]
     fn compute_hash_different_content_different_hash() {
         let hash1 = compute_hash(b"[aliases]\nb = \"make build\"\n");

crates/am/src/trust.rs

@@ -1,10 +1,11 @@
 ---
 source: crates/am/tests/snapshots.rs
+assertion_line: 584
 expression: output
 ---
 printf '%s\n' 'am: loaded .aliases'
 printf '%s\n' '  b → cargo build'
 printf '%s\n' '  t → cargo test'
 alias b="cargo build"
 alias t="cargo test"
-export _AM_PROJECT_ALIASES="b,t"
+export _AM_PROJECT_ALIASES="b|b58de66,t|ab61de4"

crates/am/tests/snapshots/snapshots__snapshot_hook_bash_with_aliases.snap

@@ -1,10 +1,10 @@
 ---
 source: crates/am/tests/snapshots.rs
+assertion_line: 613
 expression: output
 ---
 functions -e old_a
 functions -e old_b
-echo 'am: loaded .aliases'
-echo '  t → make test'
+echo 'am: .aliases changed (1 added, 2 removed)'
 alias t "make test"
-set -gx _AM_PROJECT_ALIASES "t"
+set -gx _AM_PROJECT_ALIASES "t|7f7c42c"

crates/am/tests/snapshots/snapshots__snapshot_hook_fish_transition.snap

@@ -1,10 +1,11 @@
 ---
 source: crates/am/tests/snapshots.rs
+assertion_line: 497
 expression: output
 ---
 echo 'am: loaded .aliases'
 echo '  b → cargo build'
 echo '  t → cargo test'
 alias b "cargo build"
 alias t "cargo test"
-set -gx _AM_PROJECT_ALIASES "b,t"
+set -gx _AM_PROJECT_ALIASES "b|b58de66,t|ab61de4"

crates/am/tests/snapshots/snapshots__snapshot_hook_fish_with_aliases.snap

@@ -1,10 +1,11 @@
 ---
 source: crates/am/tests/snapshots.rs
+assertion_line: 555
 expression: output
 ---
 Write-Host 'am: loaded .aliases'
 Write-Host '  b → cargo build'
 Write-Host '  t → cargo test'
 function global:b { cargo build @args }
 function global:t { cargo test @args }
-$env:_AM_PROJECT_ALIASES = "b,t"
+$env:_AM_PROJECT_ALIASES = "b|b58de66,t|ab61de4"

crates/am/tests/snapshots/snapshots__snapshot_hook_powershell_with_aliases.snap

@@ -1,10 +1,11 @@
 ---
 source: crates/am/tests/snapshots.rs
+assertion_line: 526
 expression: output
 ---
 printf '%s\n' 'am: loaded .aliases'
 printf '%s\n' '  b → cargo build'
 printf '%s\n' '  t → cargo test'
 alias b="cargo build"
 alias t="cargo test"
-export _AM_PROJECT_ALIASES="b,t"
+export _AM_PROJECT_ALIASES="b|b58de66,t|ab61de4"