Refactor manifest command to use template-based field access

Replace individual subcommands (id, version, has_server, has_webapp) with
a unified 'get {{.field_name}}' pattern that uses Go templates to access
any field from the manifest JSON dynamically.

- Update manifest.go to parse and execute Go templates against manifest data
- Update makefile calls in _setup.mk and versioning.mk to use new syntax
- Update help documentation to reflect template-based usage
- Provides more flexibility for accessing any manifest field without code changes

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

assets/build/versioning.mk

@@ -4,7 +4,7 @@
 
 # Used for semver bumping
 PROTECTED_BRANCH := master
-APP_NAME    := $(shell pluginctl manifest id)
+APP_NAME    := $(shell pluginctl manifest get '{{.id}}')
 CURRENT_VERSION := $(shell git describe --abbrev=0 --tags)
 VERSION_PARTS := $(subst ., ,$(subst v,,$(subst -rc, ,$(CURRENT_VERSION))))
 MAJOR := $(word 1,$(VERSION_PARTS))