fix(sling): reject phantom/stale dispatch under --force (gc-dv3psyz)

[vbtcl]

Problem (gc-dv3psyz)

boson/witness flagged the control-dispatcher wasting polecat sessions via three failure modes:

1. Dot-stripped phantom — a child bead ID bo-oi9tk.11 lost its dot → bo-oi9tk11, and a polecat was dispatched onto an empty branch for a bead that doesn't exist.
2. Stale-snapshot double-dispatch — an already-routed bead was re-dispatched, putting two polecats on one bead (bo-oi9tk.11: dag + rictus).
3. Resolved re-dispatch — a closed/resolved bead was dispatched again.

Root cause

internal/sling preflight treated --force as a blanket override that also disabled the bead-existence check and the fresh-state idempotency check. --force is meant to override policy (suspended agent, empty pool, cross-rig, assignee idempotency) — not correctness (does the bead exist? is it already resolved/queued?).

Fix

• Existence is validated for every real bead-ID route, even under --force. A same-rig ID that doesn't resolve is rejected (no phantom dispatch). A cross-rig bead not visible locally is still allowed under --force (the documented "remote view not yet synced" escape hatch), downgraded to a warning. Inline-text and formula routes are unaffected — they create their own bead.
• New dispatchSkipReason guard runs for plain --force bead routes and skips — as an idempotent no-op — a bead that is closed (resolved) or already routed to this exact target and awaiting pickup (the stale-snapshot / double-dispatch cases). The assignee-based idempotency that --force is designed to override is unchanged.

Acceptance criteria mapping

Criterion	Mechanism
preserve child-bead dots (no phantom)	existence check rejects dot-stripped IDs
validate bead exists before spawning	shouldValidateExistingBead runs under --force
fresh snapshot, no re-dispatch of resolved/live	dispatchSkipReason fresh-reads closed / already-queued

Tests

Rewrote the two tests that locked in the old phantom-allowing behavior; added closed-skip and already-routed-skip coverage; updated the API + CLI analogs and the --force help text.

go test ./internal/sling/ ./internal/api/ ./internal/dispatch/ ./internal/sourceworkflow/ ./cmd/gc/ — all pass. gofmt/go vet clean; full go build ./... green.

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