docs: add architecture graph

Author: GuillaumeLagrange

docs/architecture.md

@@ -0,0 +1,84 @@
+# Architecture
+
+## Execution flow
+
+This diagram shows how the CLI commands, project config, orchestrator (with providers), and executors interact during a benchmark run.
+
+```mermaid
+graph TD
+    subgraph "1. CLI Commands"
+        RUN["codspeed run <command>"]
+        EXEC["codspeed exec <command>"]
+    end
+
+    subgraph "2. Config"
+        PROJ_CFG["ProjectConfig<br/>(codspeed.yaml in repo)<br/>benchmark targets, defaults"]
+        MERGER["ConfigMerger<br/>CLI args > project config > defaults"]
+    end
+
+    subgraph "3. Orchestrator"
+        ORCH_CFG["OrchestratorConfig<br/>targets, modes, upload settings"]
+        ORCH["Orchestrator"]
+
+        PROVIDER{" "}
+        LOCAL["LocalProvider"]
+        CI["CI Providers<br/>(GitHub Actions, GitLab, Buildkite)"]
+        PROVIDER_JOIN{" "}
+
+        subgraph "Executor (per mode × per target)"
+            SETUP["1. Setup"]
+            RUN_STEP["2. Run"]
+            TEARDOWN["3. Teardown"]
+        end
+
+        UPLOAD["Upload all results to CodSpeed"]
+    end
+
+    subgraph "4. Auth"
+        CS_CFG["CodSpeedConfig<br/>(~/.config/codspeed/config.yaml)"]
+        OIDC["OIDC / env token"]
+    end
+
+    %% CLI → Config → OrchestratorConfig
+    RUN --> MERGER
+    EXEC --> MERGER
+    MERGER --> PROJ_CFG
+    PROJ_CFG -->|"merged config"| ORCH_CFG
+
+    %% CLI → Orchestrator
+    RUN -->|"single command →<br/>Entrypoint target"| ORCH_CFG
+    RUN -->|"no command + config →<br/>Exec & Entrypoint targets"| ORCH_CFG
+    EXEC -->|"always creates<br/>Exec target"| ORCH_CFG
+
+    %% Orchestrator init
+    ORCH_CFG -->|"Orchestrator::new()"| ORCH
+
+    %% Provider detection
+    ORCH -->|"auto-detect env"| PROVIDER
+    PROVIDER --> LOCAL
+    PROVIDER --> CI
+
+    %% Auth → Providers
+    CS_CFG -->|"auth token"| LOCAL
+    OIDC -->|"OIDC / env token"| CI
+
+    %% Providers → Upload
+    LOCAL -->|"token + run metadata"| PROVIDER_JOIN{" "}
+    CI -->|"token + run metadata"| PROVIDER_JOIN
+    PROVIDER_JOIN --> UPLOAD
+
+    %% Orchestrator spawns executors
+    ORCH -->|"for each target × mode:<br/>spawn executor"| SETUP
+    SETUP --> RUN_STEP
+    RUN_STEP --> TEARDOWN
+
+    %% All executors done → upload
+    TEARDOWN -->|"collect results"| UPLOAD
+```
+
+### Key interactions
+
+- **CLI → Config**: Both `run` and `exec` merge CLI args with `ProjectConfig` (CLI takes precedence). `run` can source targets from project config; `exec` always creates an `Exec` target.
+- **CLI → Orchestrator**: The merged config becomes an `OrchestratorConfig` holding all targets and modes.
+- **Orchestrator → Providers**: Auto-detects environment (Local vs CI). Local uses the auth token from `CodSpeedConfig`; CI providers handle OIDC tokens.
+- **Orchestrator → Executors**: Groups all `Exec` targets into one exec-harness pipe command, runs each `Entrypoint` independently. For each target group, iterates over all modes, creating an `ExecutionContext` per mode and dispatching to the matching executor (`Valgrind`/`WallTime`/`Memory`). After all runs complete, uploads results with provider metadata.