feat: report what the node's sb script can do as sb:<cap> on hello and heartbeat

[lr00rl]

Summary

lattice-server (integration 05c5176, PR #135) removes a deleted VPN user from an adopted line by name only on nodes that report the capability sb:user-del-by-name, and it learns script capabilities only from the agent's capability list. This branch makes the agent report them.

• With -singbox-discover on, the agent runs sb --json caps (the same -singbox-bin binary and bounded, shell-free runner as sb --json list) once at startup, before hello, and then at most every 10 minutes with a 5 s timeout.
• Each cap goes into the capabilities of the hello and metrics bodies as sb:<cap>. The server replaces its per-node set from both, so both carry it. The task poll header keeps only the agent's own capabilities.
• Only names from a fixed allowlist pass, the seven that lr00rl/sing-box v1.24.3-alpha.8 lists: user-del-by-name, user-park, user-parked-list, user-open-proxy-guard, user-match-counts, user-socks-add, user-lock (the last only on nodes with flock). Nothing else a script prints can reach the server.
• alpha.7 and older have no caps verb (the word falls through to change, which answers {"ok":false,...} with exit 1). That, a timeout, any other non-zero exit, or output that is not one {"ok":true,"caps":[...]} object means no sb: caps, one debug line, and an unchanged heartbeat. A normal log line appears only when the reported set changes.
• Second commit: runBoundedCommand now sets cmd.WaitDelay (1 s). Before, a child of sb that kept stdout open (jq, sleep, a flock wait) held the call past its deadline for as long as the child lived. This also bounds the existing sb --json list call.

capabilitiesFor, reportedCapabilities and the heartbeat goroutine are untouched, so this merges cleanly next to feat/control-plane-witness. No version bump and no changelog entry; the release cut adds those.

A probe failure on an alpha.8 node drops the sb: caps until the next probe, up to 10 minutes. That fails closed: the server keeps the old removal path for that node.

Test plan

• gofmt -l . empty, go vet ./... clean
• go test ./...: 21 packages ok
• sh scripts/check-release-workflow.sh and sh scripts/test-install-integrity.sh ok
• alpha.8 fixture is byte-identical to the fork's cmd_json_caps jq output; parse, no-flock, trim and dedupe, oversized list
• alpha.7 failure modes: exit 1 with ok=false, ok=false with exit 0, human error text, empty output, missing caps, two JSON objects, binary missing, timeout
• real shell scripts through the production runner: alpha.8, alpha.7 (exit 1), exec sleep 30, and a sleep 30 child (30.1 s before the WaitDelay fix, about 1 s after)
• allowlist drops unknown, prefixed, case-changed and comma-joined names; probe runs at startup and again only after 10 minutes; nothing runs without -singbox-discover; a heartbeat carries the prefixed caps and still withholds the durable capability while recovery is blocked
• On a node running alpha.8: the server's agent capability set shows sb:user-del-by-name after the agent restarts