reset: stop replaceSection from deleting user content after the managed section

[selimdev00]

Problem

replaceSection bounds the managed section by searching for the next ## heading:

const nextHeadingMatch = afterSection.match(new RegExp(`\\n## (?!${headingTextEscaped})`));
const after = nextHeadingMatch ? afterSection.slice(nextHeadingMatch.index ?? 0) : "";

If no following ## heading exists, after becomes "" - so everything from the marker to EOF is dropped. The managed ## BMAD-METHOD Integration block is appended at the end of the instructions file (CLAUDE.md / AGENTS.md), so any user content placed after it that isn't introduced by a ## heading is silently destroyed when the section is removed during bmalph reset:

• a trailing top-level # heading and everything under it
• plain paragraphs after the block

And because reset deletes the file entirely when the result trims to empty, a CLAUDE.md whose only ## block was BMAD could be removed outright along with that trailing content.

Fix

Bound the section by the next heading of the same or higher level (# or ##), skipping past the marker's own heading line first:

const afterMarkerLine = afterSection.slice(afterSection.indexOf("\n"));
const nextHeadingMatch = afterMarkerLine.match(/\n#{1,2} /);

• A trailing # User Notes now terminates the section and is preserved (was deleted).
• A following ## Section still terminates it (unchanged).
• ### and deeper headings remain part of the section body - matching how the shipped BMAD snippet is structured (its internal subheadings are all ###), so the block is still treated as one unit.

Existing replaceSection behavior (replace between headings, remove section, section-at-EOF, marker-not-found) is unchanged; all consumers (reset, mergeInstructionsFile) pass.

Known limitation

Headerless prose placed directly after the managed block (no #/## heading at all) is still absorbed - there's no boundary token to detect it. Fully solving that needs explicit <!-- BMAD:START/END --> delimiters around the section (a format change with migration for existing installs), which I left as a separate follow-up. This PR removes the common and most damaging cases without that migration.

Tests

Added regression cases: trailing # heading preserved on removal; ### subheadings treated as body; marker's own heading not matched as the boundary. Full unit suite passes (1838).