refactor(session): read the saved feeds through Wolfram

[ewanc26]

What this changes

The feed picker's job calls Wolfram v0.35.0's wf_agent_get_saved_feeds (pinned in the first commit; util.c joins the host Wolfram objects because the library's own moderation code now uses wf_str_dup) instead of walking the preferences and looking up generator names (~80 lines, the same as Cobalt's, which does the same in cobalt#195). Part of #53.

Verification

Host: make test (4019 checks, 0 failures). The 3DS build (devkitARM, against a Wolfram v0.35.0 build) links with no warnings. The job itself is only compiled for the 3DS, so CI and Wolfram's mock-PDS test are the checks; nothing ran on Azahar or a 3DS.

Docs

AGENTS.md and CHANGELOG updated.

Checklist

• CI is green (CI gate and flow / conventions)
• No secrets, tokens or signing keys in the diff
• Tests added or changed with the behaviour; none skipped, disabled or deleted

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