Cross-issue prose continuity: inject prior issue's prose tail + next issue's beats into the prose stage

[atomantic]

Summary

CWQE Phase 12 (#2177). Previously each issue's prose stage drafted blind to its neighbors — every issue opened and closed in a vacuum, producing the "every chapter opens/closes the same way" tic and giving the editorial loop more to fix. This wires cross-issue continuity into the prose generation stage:

• buildProseContextAugment (server/services/pipeline/textStages.js) injects the previous issue's actual closing prose (last ~2000 chars, cut on a paragraph boundary) as priorIssueProseTail and the next issue's opening beats as nextIssueBeats. Neighbor resolution is factored into a shared resolveVolumeNeighbors helper (also used by the idea-stage augment), loaded without run history.
• Prompt template (data.reference/prompts/stages/pipeline-prose.md) renders both blocks conditionally under {{#hasNeighborContinuity}} with guidance to open so it flows from the prior ending (without repeating its closing image) and land the ending so it hands off to the next issue's beats. Injected content is fenced with an "quoted context only" injection guard.
• Token-budgeted via contextBudget.js — the two blocks share a fraction of the resolved usable input budget (prior tail keeps priority), so a small-context local model degrades by trimming rather than erroring. The prior tail is trimmed from its head so the actual closing survives.
• No fabricated continuity — when the prior issue's prose doesn't exist yet (non-linear/parallel generation, or the first issue of a volume) the block simply doesn't render.
• Covers every full prose (re)generation path — manual draft, auto-runner, and Series Autopilot — because they all funnel through generateStage.
• Migration 168 rewrites the installed pipeline-prose.md for existing installs (customization-safe; customized prompts are left intact and warned). Earlier prose-touching migrations (003/027/054/127/166) and the drift baseline are resynced to the new shipped hash.

This is one slice of the multi-phase #2177 epic — the pipeline-manuscript-fix.md revision path and the Phase 7 revision loop do not yet assemble the same neighbor context (task 4 of the issue), so the issue stays open.

Test plan

• server/services/pipeline/textStages.test.js — 63 pass. New coverage: continuity injection for a middle issue, no prior tail on the first issue of a volume, prior block absent when the prior issue has no prose yet, ungrouped-issue (no season) yields no blocks, next-beats synopsis fallback, idea stage does NOT get the prose-tail blocks, extractProseTail length/boundary/absent behavior, extractNextIssueBeats precedence, and a small-context-window trim test that pins the trimmed tail keeps the CLOSING content (not the opening).
• scripts/migrations/168-prose-cross-issue-continuity.test.js and scripts/setup-data-drift.test.js pass; migrations 003/027/054/127/166 tests pass with resynced hashes.

Refs #2177