Implement post-build lifecycle command architecture

[zcourts]

Summary

This PR implements the post-build lifecycle work and aligns the command architecture with the single public developer command:

• keeps the developer-facing command surface as fission ... everywhere in public docs, examples, generated project text, and remediation guidance
• renames the tool package path to crates/tools/cargo-fission while keeping the installed binary as fission
• splits the command implementation into internal crates with explicit dependency boundaries:
  • fission-command-core
  • fission-command-run
  • fission-command-site
  • fission-command-package
  • fission-command-release
  • fission-command-ui
  • fission-credentials
• updates docs/post-build-lifecycle.md to specify the single-command UX and modular crate structure
• preserves TOML formatting/comments for release config edits
• fixes the static-site CI dependency issue by avoiding DBus-backed keyring defaults and adding dependency-boundary checks
• keeps GitHub Pages publishing using the fission command path

Verification

• cargo test -p cargo-fission -p fission-command-core -p fission-command-run -p fission-command-site -p fission-command-package -p fission-command-release -p fission-command-ui -p fission-credentials --locked
• cargo run -p cargo-fission --bin fission -- site check --project-dir documentation --release
• cargo tree -p cargo-fission --target x86_64-unknown-linux-gnu -i libdbus-sys --locked
• cargo tree -p fission-command-site --locked | rg -i 'keyring|aws-sdk|dbus|secret-service' returns no matches
• repository search confirms no public fission-cli, cargo fission, or cargo run -p cargo-fission examples remain outside implementation/workflow internals