fix(git): land the #76-#80 stack that never reached master

[ekalinin]

PRs #76, #77, #78, #79 and #80 are marked merged on GitHub, but none of their work is in
master. This branch brings all 24 commits in, without the still-open #81.

What happened

The five PRs formed a stack where each PR's base was the previous branch rather than master:

master <- chore/go-toolchain-1-26-6 <- fix/toc-absolute-paths <- feat/stdin-and-token-file
       <- feat/insert <- feat/skip-header <- fix/rate-limit-hint <- chore/docker (#81)

#82 merged chore/go-toolchain-1-26-6 into master at 10:54. Seven minutes later #76
merged its branch into chore/go-toolchain-1-26-6 - a branch that was, by then,
already behind master's future. Every subsequent merge went sideways into a neighbouring
branch, so the chain never reached master even once.

Verification that master really lacks the work: no file under cmd/ or internal/
matches insert, skip-header, no-backup or rate limit, and the packages
internal/core/usecase/insertmd, internal/core/usecase/skipheader,
internal/adapters/tokenresolver.go and internal/core/entity/marker.go do not exist there.

What this branch is

Branched from fix/rate-limit-hint (db7cdf2, the tip of #80), with origin/master
merged in. That is exactly the scope of #76-#80: 24 commits, 50 files, +2143/-85.

PR	Work
#76	fix(toc): prefix links with the document path for multiple inputs
#77	feat(cli): STDIN marker -, token.txt, fuller --version
#78	feat(insert): --insert and --no-backup
#79	feat(skip-header): the --skip-header flag
#80	fix(github): explain rate limiting in 403 and 429

The Docker work from #81 is deliberately excluded.

Nothing was lost in the original mis-merge. The five merge commits GitHub created
(b7173b5, 71d81dc, 7a079de, daee87e, abcf43b) merge a descendant into its own
ancestor and carry no unique content - the tree of feat/skip-header is byte-identical to
fix/rate-limit-hint. They are the only commits those branches hold that this one does not.

Merge conflicts resolved

Both came from #84 and were taken from master:

• .github/workflows/go.yml - keep the matrix at 1.26.6 only, drop 1.21.x
• CHANGELOG.md - keep the "requires Go 1.26 or newer" wording, add the chore(go): require Go 1.26 #84 link

Verification

go mod tidy -diff     OK
go build ./...        OK
go vet ./...          OK
golangci-lint         0 issues
go test -race -cover  ok, all 13 packages

Coverage on the packages that arrive with this branch: insertmd 98.0%, skipheader 83.9%.

Follow-up

#81 still targets fix/rate-limit-hint. Once this lands it should be retargeted to
master and rebased.

[codecov]

Codecov Report

❌ Patch coverage is 86.68942% with 39 lines in your changes missing coverage. Please review.
✅ Project coverage is 83.61%. Comparing base (870b986) to head (86946ff).

Files with missing lines	Patch %	Lines
internal/core/usecase/skipheader/skipheader.go	73.68%	7 Missing and 3 partials ⚠️
internal/adapters/filewriter.go	59.09%	5 Missing and 4 partials ⚠️
internal/adapters/stamper.go	50.00%	7 Missing ⚠️
internal/adapters/filebackup.go	72.72%	3 Missing and 3 partials ⚠️
internal/adapters/tokenresolver.go	78.57%	3 Missing ⚠️
internal/app/new.go	93.54%	1 Missing and 1 partial ⚠️
internal/core/usecase/insertmd/insertmd.go	94.87%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #85      +/-   ##
==========================================
+ Coverage   82.16%   83.61%   +1.44%     
==========================================
  Files          27       35       +8     
  Lines         673      946     +273     
==========================================
+ Hits          553      791     +238     
- Misses         77      100      +23     
- Partials       43       55      +12     

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