chore: release v0.4.0

[nikomatsakis]

🤖 New release

• symposium: 0.3.0 -> 0.4.0 (⚠ API breaking changes)

⚠ symposium breaking changes

--- failure constructible_struct_adds_field: externally-constructible struct adds field ---

Description:
A pub struct constructible with a struct literal has a new pub field. Existing struct literals must be updated to include the new field.
        ref: https://doc.rust-lang.org/reference/expressions/struct-expr.html
       impl: https://github.com/obi1kenobi/cargo-semver-checks/tree/v0.47.0/src/lints/constructible_struct_adds_field.ron

Failed in:
  field Config.agents_syncing in /tmp/.tmpvk7zqe/symposium/src/config.rs:68
  field Config.auto_update in /tmp/.tmpvk7zqe/symposium/src/config.rs:84
  field ParsedPlugin.source_name in /tmp/.tmpvk7zqe/symposium/src/plugins.rs:298
  field ParsedPlugin.source_dir in /tmp/.tmpvk7zqe/symposium/src/plugins.rs:302

--- failure derive_trait_impl_removed: built-in derived trait no longer implemented ---

Description:
A public type has stopped deriving one or more traits. This can break downstream code that depends on those types implementing those traits.
        ref: https://doc.rust-lang.org/reference/attributes/derive.html#derive
       impl: https://github.com/obi1kenobi/cargo-semver-checks/tree/v0.47.0/src/lints/derive_trait_impl_removed.ron

Failed in:
  type Output no longer derives Copy, in /tmp/.tmpvk7zqe/symposium/src/output.rs:11

--- failure enum_no_repr_variant_discriminant_changed: enum variant had its discriminant change value ---

Description:
The enum's variant had its discriminant value change. This breaks downstream code that used its value via a numeric cast like `as isize`.
        ref: https://doc.rust-lang.org/reference/items/enumerations.html#assigning-discriminant-values
       impl: https://github.com/obi1kenobi/cargo-semver-checks/tree/v0.47.0/src/lints/enum_no_repr_variant_discriminant_changed.ron

Failed in:
  variant Commands::CrateInfo 4 -> 5 in /tmp/.tmpvk7zqe/symposium/src/cli.rs:82

--- failure enum_variant_added: enum variant added on exhaustive enum ---

Description:
A publicly-visible enum without #[non_exhaustive] has a new variant.
        ref: https://doc.rust-lang.org/cargo/reference/semver.html#enum-variant-new
       impl: https://github.com/obi1kenobi/cargo-semver-checks/tree/v0.47.0/src/lints/enum_variant_added.ron

Failed in:
  variant Commands:SelfUpdate in /tmp/.tmpvk7zqe/symposium/src/cli.rs:78

--- failure function_parameter_count_changed: pub fn parameter count changed ---

Description:
A publicly-visible function now takes a different number of parameters.
        ref: https://doc.rust-lang.org/cargo/reference/semver.html#fn-change-arity
       impl: https://github.com/obi1kenobi/cargo-semver-checks/tree/v0.47.0/src/lints/function_parameter_count_changed.ron

Failed in:
  symposium::init::find_workspace_root now takes 2 parameters instead of 1, in /tmp/.tmpvk7zqe/symposium/src/init.rs:199
  symposium::plugins::load_plugin now takes 3 parameters instead of 1, in /tmp/.tmpvk7zqe/symposium/src/plugins.rs:1324

Changelog

0.4.0 - 2026-05-14

Added

• inject update nudge into session-start hook additionalContext
• add self-update command and auto-update infrastructure

Fixed

• normalize CURRENT_VERSION in test snapshots for release compatibility
• use atomic rename in mock cargo install to avoid "Text file busy"
• rustfmt formatting and switch version check to cargo search

Other

• remove binary download, default auto-update to on, add init prompt
• auto-update re-exec for both sync and hook invocations
• end-to-end auto-update re-exec with mock cargo install
• snapshot output for update check tests with expect_test
• capture Output messages, assert update warning text
• move auto-update check into cli::run, add integration tests
• move cargo override from env var to Symposium field
• cargo search, cargo_command() helper, tests, hook update behavior
• SkillOrigin keyed on source location, readable Crate dirs
• demotion to suffixed names when a new origin introduces a conflict
• promotion to unsuffixed slot when origin conflict disappears
• only suffix skill dirs with origin hash on conflict
• dispatch on group source via single match in load_skills_for_group
• SkillOrigin::Git keys on (repo, commit_sha, skill_path)
• per-group disambiguator on SkillOrigin::Plugin; sha2 hash
• introduce SkillOrigin and dedup skill installs by origin
• add has_symposium_marker helper
• address review comments
• drop unimplemented 'distribution: workspace' section
• rewrite workspace-skills page
• Document workspace skills in user guide
• Add agents-syncing: mirror user-authored skills across agent dirs

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This PR was generated with release-plz.