internal/source: Provider interface and concurrent collector

[Saber5656]

Title

internal/source: Provider interface and concurrent collector

Summary

Define the Provider interface (the future plugin boundary) and implement
the collector that runs enabled providers concurrently with per-source
timeouts, gathering events and warnings without letting any single source
fail the run (DESIGN.md §3.1, §6.1).

Context

Four providers (10, 12→via gitsrc provider, 14, 15) implement this
interface; daily/standup/sources commands call the collector. The
"sources degrade, reports never fail" property lives here.

Scope

• internal/source/provider.go, collector.go, tests with fake providers.

Detailed Requirements

1. Interface exactly per DESIGN §6.1:

   type Provider interface {
       ID() model.SourceID
       Collect(ctx context.Context, rng timeutil.Range) ([]model.Event, []model.Warning, error)
   }

2. Collect(ctx, providers []Provider, rng timeutil.Range, opts Options) Result:

   • Options{PerSourceTimeout time.Duration} (default 30s when zero);
     Result{Events []model.Event, Warnings []model.Warning}.
   • Runs each provider in its own goroutine with
     context.WithTimeout(ctx, PerSourceTimeout).
   • Timeout or context cancellation → warning
     model.WarnSourceTimeout (source_timeout) carrying the provider id;
     provider results arriving after timeout are discarded.
   • Provider error → converted to a warning
     (Code = string(id)+"_failed" unless the provider already returned
     typed warnings; the error message becomes the warning message);
     partial events returned alongside an error are kept.
   • Panic in a provider is recovered → warning (<id>_panic), run
     continues (a malformed source file must never crash the CLI —
     invariant I5).
   • Output ordering is deterministic: events sorted with
     model.SortEvents, warnings sorted by (Source, Code, Message) —
     regardless of goroutine completion order.

3. FilterByIDs(providers []Provider, ids []model.SourceID) []Provider —
   supports the --source flag (order-preserving; unknown ids are the CLI's
   problem, already validated in 07).

4. Providers receive their config at construction (each provider issue owns
   its constructor); the collector knows nothing about config.

5. Zero providers → empty Result, no warnings (the CLI decides what an
   empty report looks like).

Acceptance Criteria

• Fake-provider tests: fast+slow (slow exceeds timeout → its events
  dropped, source_timeout warning present, fast source unaffected);
  erroring provider (warning, others unaffected); panicking provider
  (recovered, warning, others unaffected); partial-events-with-error
  kept.
• Determinism: run the same mixed set 50× and assert byte-identical
  marshaled Result (ordering rule works under scheduling variance).
• Context cancellation from the caller stops in-flight providers
  (fake provider observes ctx.Done within its loop; asserted).
• -race clean.
• Package imports: stdlib + internal/model + internal/timeutil only.

Validation

go test -race -count=20 ./internal/source/ (repeat count shakes out
ordering flakes) pasted into the PR.

Dependencies

03, 04, 05 (types only — providers take typed config structs defined in 05).

Non-goals

Any real source parsing (09–15), sanitize/redact stages (18 owns them),
subprocess plugin execution (v2, ADR/DESIGN non-goal).

Design References

• docs/DESIGN.md §3.1 (pipeline), §6.1 (interface), §13 (source_timeout)

---

Source of truth: docs/issues/08-source-provider-collector.md (PR #1, branch docs/v1-design). If this issue and the repo docs disagree, the docs win. Execution order and dependencies: docs/ISSUE_PLAN.md (this is issue 08 of 33).