chore: bump version to 1.1.5

[wayyoungboy]

Summary

• bump powermem from 1.1.4 to 1.1.5
• bump powermem-mcp to 1.1.5 and pin it to powermem[server,seekdb]==1.1.5
• update the powermem-mcp README version example to 1.1.5
• add 1.1.5 to VERSION_HISTORY

Validation

• make bump-version VERSION=1.1.5
• make check-package-versions
• git diff --check
• python3.11 -m build --wheel --outdir /tmp/powermem-root-dist .
• python3.11 -m build --wheel --outdir /tmp/powermem-mcp-dist packages/powermem-mcp
• inspected wheel metadata: powermem==1.1.5 and powermem-mcp Requires-Dist: powermem[seekdb,server]==1.1.5

Release flow

• This PR only bumps versions. Publishing remains handled by the repository release CI after the release/tag is published.