feat: add await-condition task with height polling and SIGTERM action

[bdchatham]

Summary

• Adds await-condition task type for the PreInitPlan bootstrap flow. The sidecar polls the local CometBFT /status RPC at 100ms intervals until latest_block_height >= targetHeight, then optionally sends SIGTERM to seid with SIGKILL escalation after 30s.
• Introduces sidecar/rpc package (StatusClient for CometBFT status queries) and sidecar/actions package (process discovery via /proc and GracefulStop with context cancellation and isProcessGone handling).
• Adds signal.NotifyContext to the serve command for graceful sidecar shutdown on pod termination.

New packages

Package	Purpose
sidecar/rpc	CometBFT /status RPC client (500ms context timeout, no competing http.Client timeout)
sidecar/actions	Process discovery (/proc scanning, requires shareProcessNamespace: true) and graceful stop orchestration

Kubernetes notes

• Pod spec must set shareProcessNamespace: true and restartPolicy: Never
• terminationGracePeriodSeconds should exceed DefaultGracePeriod (30s) to allow the sidecar to serve the final task status
• Containers must share the same UID or seictl needs CAP_KILL
• seid must be launched directly (not via shell wrapper) for /proc/*/cmdline matching

Test plan

• Height polling: reaches target, already past target, transient RPC errors, context cancellation
• Input validation: missing/unknown condition, zero/negative/missing targetHeight, unknown action
• Target height parsing: float64, int64, json.Number
• GracefulStop: immediate exit, SIGKILL escalation, context cancellation
• Process-gone handling: ESRCH on FindPID, ErrProcessDone on SIGTERM, ESRCH on SIGKILL
• firstArg /proc cmdline parsing edge cases
• StatusClient: happy path, catching up, HTTP errors, connection refused, malformed JSON, invalid block height
• AwaitConditionTask client: validation, ToTaskRequest, JSON round-trip