feat(flow): compose flows and governed MCP tools

[gbaudrit]

Summary

Deliver the composable orchestration epic so Flows can be invoked from every supported origin, call reusable child Flows, execute governed internal or third-party MCP Tools, expose Flow-backed ToolDefinitions, model notification delivery as an ordinary Flow, and diagnose the complete causal execution path. Closes #256.

Changes

• Unify Entry, Trigger, API, MCP, Agent, and Console root Flow submission with durable origin, causation, idempotency, and WorkItem identity (Add a generic Flow call step to the Flow designer #257, Unify root Flow submission across Entry, Trigger, API, MCP, and Agent callers #260).
• Add generic Flow-call and Tool steps to the designer/runtime with exact or active version resolution, bounded nesting, cancellation, and retry-safe execution (Execute nested Flows through durable parent and child Flow Runs #258, Invoke governed MCP Tools from a generic Flow step #259).
• Add dynamic Workspace ToolDefinitions published physically by the internal Agentstration MCP and routed through the same validation/governance pipeline as third-party MCP Tools (Define dynamic Flow-backed Tools on the Agentstration MCP server #261).
• Add the bounded internal work.notification.create Tool and reusable notification delivery Flow pattern, including deterministic delivery and provider-swapping coverage (Provide reusable notification delivery Flows over internal and external MCP Tools #262).
• Add paginated causal diagnostics from root origin through child Flow Runs, Agent executions, logical Tool calls, physical attempts, providers, and governance deep links, with sensitive payloads excluded (Visualize root, child, and Tool execution causality in Flow Run diagnostics #263).
• Synchronize current main, preserve its concurrent Source Registry discovery work, and renumber the orchestration decisions as ADR-0105 through ADR-0108.
• Add English/French Console experiences, samples, automated tests, and architectural documentation.

Validation

• dotnet restore Agentstration.slnx succeeded after synchronizing current main.
• dotnet build Agentstration.slnx --configuration Release --no-restore succeeded on the synchronized stack with 0 warnings and 0 errors.
• dotnet test Agentstration.slnx --configuration Release --no-build succeeded on the synchronized stack: 831 total, 827 passed, 4 skipped optional integrations, 0 failed.
• GitHub checks restarted for the synchronized integration branch; completed checks are passing and build-and-test remains pending.
• Desktop/mobile executable smoke testing was not run; affected Console behavior has focused component rendering coverage, including the causal tree and governance links.

Breaking changes

None.