Migrate entries to current Symposium manifest schema - #21

The retired spellings this repo used no longer validate under current
Symposium: `crates = [...]` / `crates:` frontmatter is now `depends-on`,
and `[[skills]] source = "crate"` is now a `[[plugins]] source.cargo`
chained reference to the crate that ships the skills.

- dial9, error-battery-pack: crates -> depends-on; source = "crate" ->
  a chained reference. The dial9 skills actually ship in the dial9-viewer
  crate (not dial9), and error-battery-pack's ship in its own skills/;
  the references point at those.
- rust: crates = ["*"] -> depends-on = ["*"]; source.path = "." unchanged.
- assert-struct, toasty, formality-core: crates: -> depends-on:. Dropped
  the dead `activation: always` field (activation is decided by depends-on
  now; it was silently ignored).

Verified end-to-end: with a workspace depending on anyhow/toasty/
assert-struct and this repo as a registry, `cargo agents sync` installs
error-battery-pack's three skills (fetched from crates.io via the chained
reference), the two rust skills, and the assert-struct/toasty standalone
skills — while dial9 and formality-core correctly stay inactive.

rtk/rtk.toml is left as-is: it is named rtk.toml (not SYMPOSIUM.toml), so
it is not a live entry, and it wires an invasive PreToolUse Bash-rewrite
hook. Renaming it to SYMPOSIUM.toml would make it a dormant plugin
(no dependency gate) enabled by `cargo agents use rtk`; its cargo+git
installation schema should be validated at that point.