refactor: remove unused petgraph dependency from agent crates

Removes petgraph from terraphim_goal_alignment, terraphim_task_decomposition,
and terraphim_agent_registry. Analysis confirmed zero actual usage in source
code - all graph functionality is provided by terraphim_rolegraph::RoleGraph.

Architectural rationale:
- RoleGraph provides domain-specific graph optimized for semantic search
- Aho-Corasick automata integration for text matching
- Custom DFS for path connectivity (optimized for k<=8 nodes)
- Integrated document ranking built into graph traversal

Benefits:
- Reduced compilation time and dependency surface
- Clearer architectural intent
- Removed transitive dependencies (fixedbitset, etc.)

Generated with Terraphim AI

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: AlexMikhalev

Cargo.lock

@@ -40,9 +40,6 @@ log = "0.4.29"
 ahash = { version = "0.8.8", features = ["serde"] }
 indexmap = { version = "2.0", features = ["serde"] }
 
-# Knowledge graph and search
-petgraph = { version = "0.6", features = ["serde-1"] }
-
 [dev-dependencies]
 tokio-test = "0.4"
 tempfile = "3"

crates/terraphim_agent_registry/Cargo.toml

@@ -39,9 +39,6 @@ log = "0.4.29"
 ahash = { version = "0.8.8", features = ["serde"] }
 indexmap = { version = "2.0", features = ["serde"] }
 
-# Graph algorithms
-petgraph = { version = "0.6", features = ["serde-1"] }
-
 [dev-dependencies]
 tokio-test = "0.4"
 tempfile = "3"

crates/terraphim_goal_alignment/Cargo.toml

@@ -40,9 +40,6 @@ log = "0.4.29"
 ahash = { version = "0.8.8", features = ["serde"] }
 indexmap = { version = "2.0", features = ["serde"] }
 
-# Graph algorithms
-petgraph = { version = "0.6", features = ["serde-1"] }
-
 [dev-dependencies]
 tokio-test = "0.4"
 tempfile = "3"