[opencode] Add suport for HINDSIGHT_RETAIN_TAGS

[mdbenito]

This PR adds support to the opencode integration for a new environment HINDSIGHT_RETAIN_TAGS. If this variable is set, tags are added to every retain operation. I also added two missing documentation lines for HINDSIGHT_RECALL_TAGS. Incidentally, I noted that there are no tests for HINDSIGHT_RECALL_TAGS, but thought those would be better left for another PR.

(Sorry about the heavy reformatting of the table in the readme, I tried to keep it tidy as it was, but thought that it would be too wide if I adjusted all rows to match the longest one anyway, so I went for the middle ground)

[koriyoshi2041]

I reproduced the failing verify-generated-files diff locally. The functional opencode test is green; the remaining change is just prettier formatting for hindsight-integrations/opencode/README.md.

Command I ran:

cd hindsight-integrations/opencode
npx --yes prettier --write --config ../../.prettierrc.json --ignore-path ../../.gitignore .

It only changed the env-var table alignment in README.md (32 +++++++++++++++----------------), matching the CI log. Committing that formatted README should clear the generated-files check.

[mdbenito]

Command I ran:

cd hindsight-integrations/opencode
npx --yes prettier --write --config ../../.prettierrc.json --ignore-path ../../.gitignore .

Thanks! I didn't think the README would be run through prettier, although on second thought it was clear it did. Sorry for not checking first.