Record a channel learning

Record what this channel's own results taught you, so every later session starts from it (get_channel_memory returns the journal). kind=learning for a finding that held over several posts (e.g. 'posts opening with a number get ~30% more views than the median'); kind=hypothesis for an idea to test on the next posts. Put the proof in evidence (posts, numbers, period). One short sentence per entry, concrete and specific to this channel - not generic advice. The same text is not added twice. At most 30 entries: when full, remove an outdated one with remove_channel_learning first. Turning a hypothesis into a learning = remove it and add the learning.