Use etags --include to build a single TAGS file spanning all six source directories

Nobody has claimed this yet.

This issue was triaged from [`bugs/dan/1-TAGS`](https://github.com/Macaulay2/M2/blob/388c1ff0ce30d83751dea7bc7eac77fdc1305dd7/bugs/dan/1-TAGS), one of the 857 files removed from the pre-GitHub `bugs/` tree by [`d2c8d27826`](https://github.com/Macaulay2/M2/commit/d2c8d27826) and catalogued in [#36](https://github.com/Macaulay2/M2/issues/36). **The commentary below was written by Claude (Claude Opus 5, via Claude Code)**, not by @d-torrance, whose account posted it -- please weigh it accordingly.

### The original file, verbatim

```text
etags can include one tags file in another, so we could use that to make
better tag files for everything
```

### Where it stands today

Unmet, and the specific mechanism the file proposes is nowhere in the tree.

### The state today

**Six** directories build `TAGS` independently — `c/`, `d/`, `e/`, `m2/`, `system/`,
`html-check-links/` — and `m2/` additionally hand-rolls `TAGS.doc` for the Macaulay2Doc sources by
piping `find` into a shell loop (`m2/Makefile.in:43-47`). There is no `etags --include` anywhere, and no
umbrella target. So "better tag files for everything" does not exist.

### Why this is worth doing rather than marginal

The value of a tags file is jumping to a definition *wherever* it lives, and the current arrangement
gives six tables that each cover one directory — precisely the case `--include` exists for. Evidence
that the targets are used rather than vestigial: the `m2` `TAGS` rule was repaired recently, in
**#4233**, for an argument-list-too-long failure. Somebody was running it.

_{`open` · disposition `issue` · source of truth: [`bug-triage/catalog.tsv`](https://github.com/d-torrance/M2/blob/bug-triage/bug-triage/catalog.tsv)}

Inspect the existing TAGS rules in c/, d/, e/, m2/, system/, and html-check-links/, starting with m2/Makefile.in:43-47 and the recent argument-list fix in #4233. Add an umbrella target using etags --include so one TAGS file spans all six source directories, while preserving the Macaulay2Doc handling; done means the combined TAGS file builds successfully.

Written by the indexing model from the issue text.