feat: support ACP-236 auto-renewed staking commands

[anishnar]

Summary

• add platform validator add-auto-renewed for ACP-236 auto-renewed validator creation inputs
• add platform validator set-auto-config for next-cycle config updates and graceful cycle-end exit
• add platform validator reward-auto for manual RewardAutoRenewedValidatorTx submission when needed
• pin AvalancheGo dependencies to the sae-devnet-5 branch pseudo-version v1.14.3-0.20260602193739-919446e8501f (919446e8501feb23277bfaa4da660d7c03ea5150)
• adapt pkg/pchain to the APIs available in sae-devnet-5 by constructing ACP-236 unsigned txs directly instead of waiting for public wallet Issue*AutoRenewed* helpers
• keep the prior govulncheck fix with go 1.25.11 and golang.org/x/net v0.53.0

Notes

• add-auto-renewed reuses the existing permissionless validator builder to produce fee inputs and stake outputs, then repackages them as AddAutoRenewedValidatorTx.
• set-auto-config reuses the existing disable-L1 auth builder path to fetch/sign the owner auth for the original validator tx ID, then repackages it as SetAutoRenewedValidatorConfigTx.
• reward-auto initializes and issues RewardAutoRenewedValidatorTx directly through the PlatformVM client because the sae-devnet-5 wallet signer/backend marks reward-auto as unsupported.
• Source inspection of sae-devnet-5 still shows node-side execution stubs: AddAutoRenewedValidatorTx and SetAutoRenewedValidatorConfigTx return ErrUnimplemented in the standard executor, and RewardAutoRenewedValidatorTx returns ErrUnimplemented in the proposal executor. So this PR now compiles and constructs txs against the branch, but full live acceptance still depends on those AvalancheGo branch stubs being implemented on the devnet node side.

Closes #20

Tests

• go test ./...
• GOTOOLCHAIN=go1.25.9+auto go run golang.org/x/vuln/cmd/govulncheck@v1.2.0 ./...
• go run . validator add-auto-renewed --help
• go run . validator set-auto-config --help
• go run . validator reward-auto --help