write-safety: generalize plan/apply gating to all mutations

[JacobPEvans-personal]

Goal: Every mutating command shares one safe path.
Method: extract the index-create gating into a shared write helper — dry-run preview + target fingerprint; risk tiers (safe/mutating/destructive) with an allowlist and blast-radius caps; confirmation + --yes; append-only audit; server-side validate where supported.
Acceptance: all write commands route through it; destructive ops require explicit opt-in; a test per gate.

[JacobPEvans-personal]

Progress from #1 (merged in 25802ba): the gating primitives exist and are wired into index create — output.confirm_write (dry-run / --yes / non-interactive fail-fast), client.write dry-run preview, and the append-only audit log. Still open: extract these into one shared write helper used by every mutation, with risk tiers (safe / mutating / destructive), blast-radius caps, and server-side validate.

[JacobPEvans-personal]

Slice landed (0.2.0 leg). PR #23 extracted the single do_write gate (gate → run → audit) from the five index writes, and every mutation added since routes through it: index CRUD, saved searches, KV Store records (#33), platform restart/settings (#34), app install + deploy (#36), and HEC/datamodel/lookup writes (#37). One seam, so any later tier is a local change.

Still open — the full framework (add each only when a real second writer or the Lambda needs it; trigger noted):

• Plan tokens / two-step apply — trigger: a write whose preview must be approved out-of-band.
• Target fingerprinting + optimistic concurrency — trigger: concurrent writers to the same object.
• Risk-tier allowlists + blast-radius caps — trigger: bulk/fleet writes (e.g. the scheduled-search skew in feat: skew scheduled saved searches off default cron times (owner+app scoped) #35).
• VCT_SPLUNK_READONLY freeze-writes kill switch — trigger: a shared/agent deployment that must be globally read-only (a ~2-line add inside do_write).
• Server-side per-resource validate — trigger: lands with config: conf files, properties, and validation #7 (config/validation).

Keeping this issue open to track the framework; the gate itself is done.