docs(release): ship on a weekly Monday train

[datlechin]

Why

Releases have no fixed day or soak: about a quarter of tags landed on a weekend, and several regressions shipped straight to stable. This writes down the weekly Monday train approved on 2026-10-19.

Change

• RELEASING.md, new Schedule section: the Monday order (promote, forward-port, cut the next beta.1), the promotion gate, the slip rule, beta.N and patches Tuesday to Thursday with the patch bar, no weekend tags, the year-end and Tet skips, beta issue triage.
• Branches: a line closes when X.(Y+1).0 ships; a later security or data-loss fix ships as a critical patch on the current stable line.
• Plugins: a TABLEPRO_PLUGINKIT_VERSION bump enters a line only at beta.1.
• Release skill: section 2 reads the newest beta's age, section 3 decides by day and points to Schedule, section 6 follows the patch bar and the closed line. Phasing stays 21600 s.

Verified

• verify.sh agent-docs passes.