Fix web preview proxy hardening

[Just-Insane]

Description

Follow-up to #98 for the unresolved web-preview review threads:

• Replace the unsupported external _redirects proxy with a Pages Function under /api/*.
• Keep Cloudflare deploy secrets out of the checkout/install/build steps; only config/deploy steps receive deploy credentials.
• Disable checkout credential persistence and pass GH_TOKEN only to the comment step.
• Generate the preview comment body without leading Markdown indentation.

Related to #98:

• Spec 16 web frontend and deploy leg #98 (comment)
• Spec 16 web frontend and deploy leg #98 (comment)
• Spec 16 web frontend and deploy leg #98 (comment)
• Spec 16 web frontend and deploy leg #98 (comment)

Fixes #

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• pnpm lint, pnpm fmt:check, pnpm typecheck, pnpm test:coverage, pnpm knip, and pnpm build all pass locally

Validation run for this workflow-only change:

• actionlint .github/workflows/web-preview.yml
• pnpm fmt:check
• node --check 'functions/api/[[path]].js'
• npx --yes wrangler@4.107.1 pages functions build functions --outfile /tmp/charm-pr98-web-preview-functions.js

AI disclosure:

• Partially AI assisted (clarify which code was AI assisted and briefly explain what it does).
• Fully AI generated (explain what all the generated code does in moderate detail).

Codex assisted with the workflow hardening and Pages Function proxy follow-up. The function forwards same-origin /api/* preview requests to the configured companion API base at runtime, keeping authenticated preview traffic same-site for the browser while avoiding unsupported _redirects external proxying.

[Copilot]

Pull request overview

This PR hardens the Cloudflare Pages “web preview” pipeline by moving the same-origin /api/* proxying logic into a Pages Function and tightening GitHub Actions secret exposure and commenting behavior.

Changes:

• Add a Cloudflare Pages Function at functions/api/[[path]].js to proxy same-origin /api/* requests to a configured companion API base URL at runtime.
• Harden .github/workflows/web-preview.yml by scoping Cloudflare credentials to only the configuration/deploy steps, disabling checkout credential persistence, and passing GH_TOKEN only to the PR comment step.
• Generate the preview comment body without leading indentation to avoid unintended Markdown formatting.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
functions/api/[[path]].js	Introduces the /api/* Pages Function proxy used by web previews.
.github/workflows/web-preview.yml	Restricts credential exposure and improves preview comment generation.

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[Copilot]

Pull request overview

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

[chatgpt-codex-connector]

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Reviewed commit: e24117a609

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[Copilot]

Pull request overview

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

[Copilot]

Pull request overview

Copilot reviewed 2 out of 2 changed files in this pull request and generated 3 comments.

[Copilot]

Pull request overview

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

[Copilot]

Pull request overview

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

[Copilot]

Pull request overview

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

[Copilot]

Pull request overview

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

[Copilot]

Pull request overview

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

[chatgpt-codex-connector]

💡 Codex Review

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Reviewed commit: 6c4c71a124

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[chatgpt-codex-connector]

💡 Codex Review

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Reviewed commit: 3b1ffff10f

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Your team has set up Codex to review pull requests in this repo. Reviews are triggered when you

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• Mark a draft as ready
• Comment "@codex review".

If Codex has suggestions, it will comment; otherwise it will react with 👍.

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[chatgpt-codex-connector]

Allow slash-dot MXID localparts

The fresh evidence in this revision is that decodedSegment.split(/[\\/]) still treats an encoded slash inside a single route parameter as path hierarchy. When a valid Matrix user ID localpart contains /./ or /../ (the Matrix grammar allows both . and / as opaque localpart characters), matrixTransport sends it as one percent-encoded segment such as /api/presence/%40a%2F.%2Fb%3Aexample.org, but this check returns 400 before the companion sees it; since the target URL keeps %2F encoded, this blocks legal MXIDs rather than preventing traversal.

Useful? React with 👍 / 👎.