feat(compass): remove dead initial_prompt from the agent-session start path

[seal-agent]

This PR is part of a stack containing 2 PRs:

1. main
2. fix(runner): repair errorResult callsite skew breaking main build #294
3. "feat(compass): remove dead initial_prompt from the agent-session start path" (this PR)

The initial_prompt field on StartAgentSessionRequest, SpawnAgentRequest, and the internal SpawnPeerRequest was accepted on the wire but never delivered to the agent — dead plumbing removed across the proto, all four generated trees, the go/server relay callers, and the compass-agent spawn tool; the freed field numbers and names are reserved for forward-compat hygiene. The go/e2e package is intentionally held (does not compile against the dropped field) pending a turn-driver ownership ruling.

Spec-impact: none

Refs SEA-1959

Co-authored-by: Matt Wilkinson matt@sealedsecurity.com

[linear-code]

SEA-1959

[seal-agent]

Review loop complete — review-clean, ready for merge gate. (head df739662)

The mandatory review agent ran two rounds:

• Round 1 (head cbbf0be5): 1 medium + 1 low, both auto-fixed. Medium — go/e2e/legthreefour_test.go spawn tool-call JSON literal still hard-coded initial_prompt (build/runtime-harmless: a Go string literal, and arktype spawnParameters defaults to onUndeclaredKey:"ignore", so the key is silently dropped — which is why the -tags podman build passed — but it reintroduced the exact misleading surface this PR removes). Low — a stale StartSession(initial_prompt) doc comment in legtwo_test.go.
• Round 2 (head 2fb5aa43): all-clear at the high+medium floor. Every lens PASS — proto reserved-hygiene (both field number and name reserved for all 3 messages), gen consistency (rawDesc bytes verified at byte level), go/server clean removal, TS lifecycle schema/mapping/test, wire-contract compatibility. One low (a drift-prone lifecycle.ts:79-93 line-span citation), also fixed → head df739662.

Every finding dispositioned (all fixed, none deferred). The strip is complete: zero live initial_prompt/initialPrompt/InitialPrompt references remain in non-generated source (only proto reserved entries + their generated rawDesc encoding).

Stack: #295 is stacked on #294 (base compass-server-fix-dispatch-errorresult-skew); jj-vine re-points the base to main automatically when #294 merges. Verified against #294's fixed tree: go build -tags podman ./... + untagged go build ./... exit 0, go vet -tags podman ./e2e/... exit 0, golangci 0, gofmt clean, full gated moon ci clean (32 tasks, 0 failed).