feat: Add background execution with --background flag and run management

[jahvon]

Add support for detaching executable runs into background processes and
managing them through the logs command. This enables long-running tasks
to execute without blocking the terminal.

• flow exec <ref> --background spawns a detached child process and
  returns a short run ID immediately. Output is captured in the log
  archive while stdout/stderr are redirected to /dev/null.
• flow logs --running lists active background processes with the same
  output/TUI patterns as regular execution history (yaml, json, tui).
  Stale entries whose PIDs are no longer alive are pruned automatically.
• flow logs kill <run-id> sends SIGTERM to terminate a background run.
• flow logs attach <run-id> tail-follows the log archive file,
  streaming output in real time until the process exits or Ctrl-C.

Implementation details:

• BackgroundRun model and CRUD methods added to DataStore (new "background"
  BBolt bucket) with mock support.
• Child process eagerly links its log archive path into the run record so
  attach/streaming works while execution is in progress.
• Archive file lookup uses direct filename scan (not ListArchiveEntries)
  to avoid the empty-file skip that would block early linking.
• Structured output (json/yaml) returns empty arrays instead of message
  strings for parsability.
• E2E tests cover --running (empty, active, stale pruning) and attach.
• Documentation added to execution-history guide and CLI reference pages.

[codecov]

Codecov Report

❌ Patch coverage is 36.50794% with 200 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
cmd/internal/exec.go	3.94%	70 Missing and 3 partials ⚠️
internal/io/logs/background.go	20.77%	60 Missing and 1 partial ⚠️
cmd/internal/logs.go	54.86%	41 Missing and 10 partials ⚠️
pkg/store/store.go	72.09%	6 Missing and 6 partials ⚠️
internal/io/logs/output.go	50.00%	3 Missing ⚠️

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