feat: add --list-tasks and --start-at-task for inspection and resume

[josegonzalez]

Summary

--list-tasks on apply and plan walks the resolved task plan post --play / --tags filtering and post loop: expansion and prints one line per envelope without running. when: predicates evaluate against inputs only; predicates that reference .registered render as [unknown].

--start-at-task <name> on apply skips earlier tasks (rendered as [skipped] ... (before --start-at-task)) and runs from the matched task onward, including descending into a block: group when the match lives inside.

validate --strict gains cross-reference checks for --play and --start-at-task, surfacing unknown_play_reference / unknown_start_at_task problems for typos before they reach apply.

Closes #212.

[dokku-bot]

sh-checker report

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shfmt errors

'shfmt -l -d -i 2' returned error 1 finding the following formatting issues:

----------
tests/bats/list_resume.bats
diff tests/bats/list_resume.bats.orig tests/bats/list_resume.bats
--- tests/bats/list_resume.bats.orig
+++ tests/bats/list_resume.bats
@@ -57,7 +57,7 @@
   assert_output --partial "docket-test-list-1"
   assert_output --partial "docket-test-list-2"
   run dokku apps:exists docket-test-list-1
-  assert_failure   # never created
+  assert_failure # never created
 }
 
 @test "--list-tasks honors --tags filter" {
@@ -133,7 +133,7 @@
   assert_output --partial "[skipped] first"
   assert_output --partial "before --start-at-task"
   run dokku apps:exists docket-test-start-at-first
-  assert_failure   # skipped
+  assert_failure # skipped
   run dokku apps:exists docket-test-start-at-second
   assert_success
   run dokku apps:exists docket-test-start-at-third
----------

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