feat(tracker): add Tracker trait with Linear and GitHub Projects V2 impls

- New arc-tracker crate defines the Tracker trait and shared Issue/BlockerRef types
- arc-linear: replace Issue/BlockerRef with arc-tracker re-exports, add LinearTracker struct implementing Tracker, remove free functions
- arc-github: add GitHubTracker implementing Tracker via Projects V2 GraphQL API, with lazy project node ID resolution (org → user fallback) and create_installation_access_token_for_projects helper
- 47 tests in arc-github (15 new), 24 tests in arc-linear (updated to use LinearTracker)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Cargo.lock

@@ -303,6 +303,8 @@ dependencies = [
 name = "arc-github"
 version = "0.2.0"
 dependencies = [
+ "arc-tracker",
+ "async-trait",
  "base64",
  "chrono",
  "jsonwebtoken",
@@ -318,6 +320,8 @@ dependencies = [
 name = "arc-linear"
 version = "0.2.0"
 dependencies = [
+ "arc-tracker",
+ "async-trait",
  "mockito",
  "reqwest 0.12.28",
  "serde",
@@ -439,6 +443,13 @@ dependencies = [
  "tracing",
 ]
 
+[[package]]
+name = "arc-tracker"
+version = "0.2.0"
+dependencies = [
+ "async-trait",
+]
+
 [[package]]
 name = "arc-types"
 version = "0.2.0"

lib/crates/arc-github/Cargo.toml

@@ -9,12 +9,15 @@ description = "GitHub App authentication and API helpers for Arc"
 doctest = false
 
 [dependencies]
+arc-tracker = { path = "../arc-tracker" }
+async-trait.workspace = true
 serde.workspace = true
 serde_json.workspace = true
 reqwest.workspace = true
 jsonwebtoken.workspace = true
 chrono.workspace = true
 tracing.workspace = true
+tokio = { workspace = true }
 
 [dev-dependencies]
 mockito = "1"

lib/crates/arc-linear/Cargo.toml

@@ -9,6 +9,8 @@ description = "Linear issue tracker API helpers for Arc"
 doctest = false
 
 [dependencies]
+arc-tracker = { path = "../arc-tracker" }
+async-trait.workspace = true
 serde.workspace = true
 serde_json.workspace = true
 reqwest.workspace = true

lib/crates/arc-tracker/Cargo.toml

@@ -0,0 +1,12 @@
+[package]
+name = "arc-tracker"
+edition.workspace = true
+version.workspace = true
+license.workspace = true
+description = "Tracker trait and types for issue tracking integrations"
+
+[lib]
+doctest = false
+
+[dependencies]
+async-trait.workspace = true

lib/crates/arc-tracker/src/lib.rs

@@ -0,0 +1,57 @@
+use async_trait::async_trait;
+
+#[derive(Debug, Clone)]
+pub struct BlockerRef {
+    pub id: String,
+    pub identifier: String,
+    pub state: String,
+}
+
+#[derive(Debug, Clone)]
+pub struct Issue {
+    /// Provider-native issue node ID.
+    pub id: String,
+    /// Provider-native project-item ID for status updates.
+    /// None for providers where the issue ID is sufficient (e.g. Linear).
+    /// For GitHub Projects, this is the ProjectV2Item node ID.
+    /// Each Tracker impl is scoped to a single project, so this is unambiguous
+    /// even when an issue belongs to multiple project boards.
+    pub project_item_id: Option<String>,
+    /// Human-readable identifier (e.g. "ABC-123" or "#42").
+    pub identifier: String,
+    pub title: String,
+    pub description: Option<String>,
+    pub priority: Option<i32>,
+    pub state: String,
+    pub branch_name: Option<String>,
+    pub url: String,
+    pub assignee_id: Option<String>,
+    pub labels: Vec<String>,
+    pub blocked_by: Vec<BlockerRef>,
+    pub created_at: Option<String>,
+    pub updated_at: Option<String>,
+}
+
+/// Unified interface for project management / issue tracking systems.
+///
+/// Implementations are constructed with provider-specific config and used as
+/// `Arc<dyn Tracker>` or `Box<dyn Tracker>`.
+#[async_trait]
+pub trait Tracker: Send + Sync {
+    /// Return the authenticated user's ID in the provider's system.
+    async fn fetch_viewer_id(&self) -> Result<String, String>;
+
+    /// Add a comment to an issue. Each impl extracts the appropriate ID.
+    async fn create_comment(&self, issue: &Issue, body: &str) -> Result<(), String>;
+
+    /// Transition an issue to a new state by name.
+    /// Each impl extracts the appropriate ID (issue ID or project item ID).
+    async fn update_issue_state(&self, issue: &Issue, state_name: &str) -> Result<(), String>;
+
+    /// Fetch issues matching any of the given state names.
+    /// Project identity is in the impl's config, not here.
+    async fn fetch_candidate_issues(&self, state_names: &[&str]) -> Result<Vec<Issue>, String>;
+
+    /// Fetch specific issues by their provider-native IDs (`Issue::id` values).
+    async fn fetch_issues_by_ids(&self, ids: &[&str]) -> Result<Vec<Issue>, String>;
+}