fix(#4398): render the pending-todo bullet link repo-relative (#4416)

* test(#4384): failing-first regression rows for the macOS long-base todo-cap failure

The 240-char pending-todo bullet cap must be deterministic w.r.t. where the
repo is checked out. Deterministic long-base-path fixtures (a single 110-char
segment, no real macOS dependency) reproduce next's own macos shard 3/3
failure (run 34038716700) on every OS: with an absolute link the bullet
exceeds the cap and the documented needs-first truncation drops the
'Needs <solution>' clause. Rows cover the determinism property (byte-identical
bullets under short and long bases), the CLI surface, relative-path stability,
legacy no-projectRoot behavior, drop-order preservation, and adversarial
edges (outside-root, path===root, non-string path).

* fix(#4384): render the pending-todo bullet link repo-relative

renderPendingTodosMarkdown gains an optional projectRoot; when given and the
todo's path is absolute, the bullet's [todo file](…) target becomes
toPosixPath(path.relative(projectRoot, path)) — the idiom already used for
project_exists. cmdInitTodos passes cwd.

The JSON todos[].path field stays absolute (#2376). Only the rendered display
link changes: embedding the machine-variable absolute base let macOS's
/private/var/folders/… temp paths consume the 240-char budget and drop the
'Needs' clause on long-path machines only — next's own macos-latest shard 3/3
went red on exactly this (run 34038716700), Linux's short /tmp passed. The
240-char whole-bullet cap and the needs→title→area drop order are unchanged;
this matches PR #4384's own canonical example, docs, and unit tests, which all
show repo-relative links. Docs updated at all three surfaces that describe the
bullet (COMMANDS.md, templates/state.md, reference/state-md.md — the last was
still pre-#4384 'count and reference' prose).

Fixes the macOS regression introduced by #4384; next is red on its own CI.

* test(#4384): fix substring false positive in the outside-root regression row

The ../-form relative link legitimately contains the absolute path as a
substring, so !line.includes(absolutePath) fired on correct output (caught by
the first remote verify run, linux-node24 44018/44019). Assert the property
itself instead: extract the link target and require it to be non-absolute and
not equal to the absolute path.

* chore(#4398): backfill PR number in changeset

---------

Co-authored-by: sim <sim@local>

docs/reference/state-md.md

@@ -224,7 +224,7 @@ Updated after each plan completion.
 
 **Decisions** — a summary of recent decisions affecting current work (full log lives in `PROJECT.md`). Added via `gsd-tools state add-decision`.
 
-**Pending Todos** — count and reference to `.planning/todos/pending/`. Captured via `/gsd-capture`.
+**Pending Todos** — one bullet per pending todo (`- [date] [area] title — [todo file](repo-relative path) — Needs ...`, capped at 240 characters; repo-relative link keeps the cap independent of checkout path length). Captured via `/gsd-capture`.
 
 **Blockers/Concerns** — issues affecting future work, prefixed with the originating phase. Added via `gsd-tools state add-blocker`; resolved via `gsd-tools state resolve-blocker`.