[Backport 2026.1] feature(nemesis): Add precheck() hook to NemesisBaseClass - #15699

… the run loop

Add NemesisRunner.precheck_nemesis(), invoked once at the start of run() before
the execution loop. It iterates disruptions_list, evaluates each nemesis's
precheck(), prunes the infeasible ones in place, and returns the (name, reason)
exclusions. Each exclusion is reported exactly once:
- a returned reason -> DisruptionEvent.skip() (SKIPPED, NORMAL) and Argus
  finalize_nemesis(status=SKIPPED)
- a raised exception -> DisruptionEvent.add_simple_error() (FAILED, ERROR) and
  Argus finalize_nemesis(status=FAILED) -- a broken precheck() is a test error,
  not an intentional skip

Argus reporting reuses argus_submit(), which gains an optional node parameter so
build-time callers can pass cluster.nodes[0] as the representative node (no
target_node exists yet); existing call-sites keep the self.target_node default.

run() calls precheck_nemesis() before the loop and, when exclusions emptied the
rotation, publishes one CRITICAL InfoEvent naming each excluded nemesis and
returns. build_disruptions_by_selector() stays a pure builder returning a plain
list, so runners that legitimately keep an empty disruptions_list and override
call_next_nemesis() (NoOpMonkey, ManagerRcloneBackup, ManagerNativeBackup,
CategoricalMonkey) are unaffected.

Tests: add precheck() stubs to TestBaseClass and TestExecuteBaseClass (required
to keep run()-based tests green now that run() calls precheck()), add
PrecheckSkipNemesis/PrecheckErrorNemesis, and add execute_nemesis/test_precheck.py
covering pruning, DisruptionEvent shapes, Argus submit/finalize, and the run()
empty-rotation path (skip-vs-exception variants parametrized, full event dicts
asserted).

(cherry picked from commit d98f803)

Add a 'Pre-execution skip checks (precheck)' section to docs/nemesis.md covering
the contract, what belongs in precheck() vs disrupt() (plain language, no
internal Category taxonomy), the representative-node rule (cluster.nodes[0]),
the SKIPPED/FAILED reporting behavior, the empty-rotation CRITICAL behavior, and
a before/after migration example. Update the build_disruptions_by_selector runner
example accordingly.

Update the writing-nemesis skill and the AGENTS.md nemesis section with the
precheck() contract and category rule. Advance the plan status to in_progress,
rewrite its Commit Plan section to match the final precheck_nemesis() design,
and regenerate the progress roadmap asset.

(cherry picked from commit 964eceb)

(cherry picked from commit 82b64e2)