Release Signboard 1.6.0

[cdevroe]

Summary

Prepares the Signboard 1.6.0 release branch.

• Adds opt-in local Ollama Smart Card Actions for titles, summaries, task lists, auto-labeling, smart paste, and custom prompts.
• Expands date workflows with card start dates, task start/scheduled markers, compact Dates controls, Planner/date filter handling, and CLI/MCP date metadata.
• Upgrades Table view with list filtering, sorting, checkbox/range selection, and bulk archive/move/label/date actions.
• Improves board operations with duplication, faster snapshot-based board reads, atomic file writes, safer transactional ordering, archive/list restore coverage, and external-change refresh behavior.
• Updates CLI and MCP board discovery, board creation, imports, dry-run card writes, timestamp metadata, docs, and agent guidance for the 1.6.0 surface.
• Bumps the app version to 1.6.0 and refreshes release-facing docs.

Impact

Users get a larger planning and automation release centered on dated work, AI-assisted card editing, dense Table management, safer local-file operations, and better agent/CLI workflows. Agents get more reliable board discovery and richer metadata without requiring users to hand-type board paths.

Validation

• Not run as part of this PR drafting step.
• Branch includes updated unit/smoke coverage for Smart Actions, app settings, board duplication, board snapshots, card ordering, CLI/MCP workflows, date/task parsing, imports, and Playwright smoke coverage.