Bump workspace version to 0.5.10 and refresh docs.
Bundles the 7 fixes merged on main since v0.5.9:
- #1034 auth fail-closed (#1071)
- #980 channel agent name prefix (#1072)
- #1043 multimodal text+images (#1073)
- #809 openfang hand config subcommand (#1074)
- #843 context.md re-read per turn (#1075)
- #905 config get default_model.base_url (#1076)
- #1069 scheduler unification and migration (#1077)
README: fix stale 0.3.30 badge and March 2026 header to 0.5.10 and April 2026,
drop em dashes throughout.
CHANGELOG: new 0.5.10 section with the above, plus notes on #818 and #819
which were closed as invalid.