feat(make): add makefile help and idiomatic cleanup

[cds-amal]

You can make it if you try

Add a make help target that uses an awk script to parse structured comments in the Makefile itself. ## comments describe individual targets; ### comments act as section headers. The output is grouped into four sections: Build, Test, Code quality, and Graph generation. Internal helper targets (like check-graphviz and rustup-clear-toolchain) are intentionally left without ## comments so they stay hidden from the help output.

Idiomatic fixes applied across the board:

• .DEFAULT_GOAL := build replaces the old default: build target. The old approach created a real target called default that showed up in tab-completion and could collide with other tooling; .DEFAULT_GOAL is the built-in make mechanism for this.

• $(MAKE) replaces bare make in the golden target. When make is invoked with flags (e.g., make -j4), a bare make in a recipe starts a fresh make process that loses those flags and, critically, the jobserver file descriptors. $(MAKE) preserves both.

• .PHONY is now declared immediately above every phony target. A phony target is one that doesn't produce a file of the same name (i.e., make build doesn't create a file called build). Without the declaration, if a file named build or clean happened to exist, make would see it as up-to-date and skip the target entirely. Previously several targets (build, clean, golden, format, style-check) were missing their declarations.

• Variable references normalized from ${VAR} to $(VAR). Both work identically in GNU make, but $(VAR) is the conventional style. ${VAR} is visually ambiguous with shell variable expansion inside recipes, so reserving curly braces for shell context (like $${rust}) and parens for make context makes it easier to tell which layer is doing the expansion.

• TOOLCHAIN_NAME='' fixed to TOOLCHAIN_NAME=. Unlike in shell, make does not interpret quotes as delimiters: the old value was literally the two-character string '', not empty. This happened to be harmless (rustup would just fail to find a toolchain named '') but was misleading.

• Removed the ECHO_CMD=echo variable, which was only used once and added indirection with no benefit. Replaced with a direct @echo.

[Copilot]

Pull request overview

Adds a self-documenting make help target to expose supported developer workflows directly from structured comments in the Makefile, while also tightening Make idioms to avoid common invocation and phony-target pitfalls.

Changes:

• Added a help target that parses ## (target descriptions) and ### (section headers) from Makefile via awk and prints grouped help output.
• Replaced default: build with .DEFAULT_GOAL := build and switched recursive make invocation to $(MAKE) for jobserver/flag preservation.
• Normalized Make variable references and expanded .PHONY declarations across targets; fixed TOOLCHAIN_NAME’s empty default value.

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[cds-amal]

Replaced by #142