LCORE-2664: add BYOK Confluence auto-import spike and spec docs

Design spike for LCORE-788 (auto import of content for BYOK from
Confluence; scope reduced 2026-06 to Confluence only) under the epic
readout LCORE-2652.

The spike doc records:
- Settled decision S1: the importer lives in lightspeed-core/rag-content,
  following the byok-pdf precedent (Decision 3); the PoC required zero
  rag-content library changes.
- Strategic decisions S2-S5 for PM confirmation: periodic full re-crawl +
  rebuild + rollout as the refresh baseline (no hot-reload), DB file as
  the primary artifact with OCI image optional, a Kubernetes CronJob
  reference manifest as the deployable automation, and no new
  lightspeed-stack configuration surface.
- Technical decisions T1-T8 for tech-lead confirmation: fetch client
  (atlassian-python-api), v1 body.export_view rendered HTML, Cloud
  API-token + DC PAT auth, space/CQL selection, attachments out of scope,
  manifest-driven citation metadata, embedding-model pinning, and
  CQL-delta + version-skip + ID-enumeration-diff change detection.
- PoC results from a real Confluence Cloud crawl (20-page space,
  read-only token): end-to-end build, correct retrieval with per-page URL
  citations, incremental re-run with zero re-fetches/re-embeds.
- Proposed JIRAs: one epic with six tickets (e2e feature files first,
  step definitions, fetch stage, importer command + packaging, CronJob
  manifest + admin guide, lightspeed-stack docs) plus one incidental
  bug ticket; parses cleanly with dev-tools/file-jiras.sh.

The spec doc is the permanent reference: requirements R1-R10, use cases,
architecture (fetch stage -> existing pipeline -> artifact -> CronJob
refresh), acceptance test surface driving the e2e kickoff ticket, and
implementation suggestions with concrete rag-content insertion points.

Author: max-svistunov